Unlocking the Secrets of Stainless Steel: Master the Professional Knowledge

What is Stainless Steel?

1. Definition:

Stainless steel is a high-alloy steel that can resist corrosion. It has a beautiful surface and does not require surface treatment such as plating or painting.

Therefore, many machinery factories often do not perform any painting treatment to indicate that their products are made of stainless steel, while black materials (commonly known as regular steel) must undergo anti-rust treatment.

Steel that can resist weak corrosive media such as air, steam, water, or has stainless properties is called stainless steel. The steel that can resist chemical corrosive media (such as acid, alkali, salt, etc.) is called acid-resistant steel.

In practical applications, the steel that can resist weak corrosive media is often referred to as stainless steel, while the steel that can resist chemical corrosive media is called acid-resistant steel.

Due to the differences in chemical composition, the former may not resist chemical corrosion, while the latter generally has stainless properties. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.

2. Characteristics:

The most basic characteristic of stainless steel is its rust resistance under atmospheric conditions and corrosion resistance in various liquid media. This characteristic is directly related to the chromium content in the steel.

The presence of chromium forms an invisible film on the surface of the steel, which can prevent oxidation and make the material “passive” or resistant to corrosion. The addition of other elements such as nickel or molybdenum can improve the corrosion resistance, strength, and temperature resistance.

Corrosion resistance increases with the increase of chromium content.

When the chromium content reaches 10.5% or higher, the steel undergoes a mutation in this characteristic from rust-prone to rust-resistant, and from corrosion-susceptible to corrosion-resistant.

Moreover, as the chromium content increases beyond 10.5%, the rust and corrosion resistance of the steel continue to improve.

The highest chromium content of general stainless steel is 26%, and higher chromium content is not necessary.

3. Classification:

There are many types of stainless steel with varying properties. Common classification methods include:

Classification based on the structure of the steel, such as martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, and duplex stainless steel.

Austenitic Stainless Steel:

The matrix is mainly austenite structure (CY phase) with face-centered cubic crystal structure, non-magnetic, and strengthened mainly by cold working (which may result in some magnetism). It is identified by numbers in the 200 and 300 series, such as 304, by the American Iron and Steel Institute.

Ferritic Stainless Steel:

The matrix is mainly ferrite structure (α phase) with body-centered cubic crystal structure, magnetic, generally cannot be hardened by heat treatment, but can be slightly strengthened by cold working. It is identified by numbers such as 430 and 446 by the American Iron and Steel Institute.

Martensitic Stainless Steel:

The matrix is martensitic structure (body-centered cubic or cubic), magnetic, and can adjust its mechanical properties by heat treatment. It is identified by numbers such as 410, 420, and 440 by the American Iron and Steel Institute.

At high temperatures, martensite has an austenitic structure. When cooled to room temperature at an appropriate rate, the austenitic structure can transform into martensite (quenching).

Duplex Austenitic-Ferritic (Dual-Phase) Stainless Steel:

The matrix has both austenitic and ferritic structures, with a higher content of the less phase matrix generally greater than 15% and magnetic. It can be strengthened by cold working, and 329 is a typical duplex stainless steel.

Compared with austenitic stainless steel, duplex steel has higher strength and significant improvements in intergranular corrosion resistance, chloride stress corrosion resistance, and pitting corrosion resistance.

Precipitation Hardening Stainless Steel:

The matrix is austenitic or martensitic structure and can be hardened by precipitation hardening treatment. It is identified by numbers in the 600 series, such as 630 (17-4PH), by the American Iron and Steel Institute.

Classification based on the steel’s main chemical composition or some characteristic elements, such as chromium stainless steel, chromium-nickel stainless steel, chromium-nickel-molybdenum stainless steel, ultra-low carbon stainless steel, high-aluminum stainless steel, high-purity stainless steel, etc.

Classification based on the steel’s performance characteristics and uses, such as nitric acid-resistant (nitric acid grade) stainless steel, sulfuric acid-resistant stainless steel, pitting corrosion-resistant stainless steel, stress corrosion-resistant stainless steel, high-strength stainless steel, etc.

Classification based on the steel’s functional characteristics, such as low-temperature stainless steel, non-magnetic stainless steel, easy-to-cut stainless steel, superplastic stainless steel, etc.

Currently, the most commonly used classification methods are based on the steel’s structural and chemical composition characteristics, or a combination of the two.

4. Forms of Stainless Steel:

Stainless steel can be divided into five major categories: coil, sheet, profile, pipe, and fittings. The most important forms are coil and sheet.

Profiles are materials of various shapes made from sheets, including angle steel, flat steel, I-beam, and channel steel.

Steel pipe mainly refers to seamless steel pipes, which differ from welded pipes in that they are formed in one piece.

Fittings mainly refer to elbows, flanges, and other small components.

Coil and sheet are essentially the same, just with different forms when leaving the factory. Coil is rolled into a roll while sheet is in a flat form.

If it leaves the factory as sheet, it is also called the original plate and is generally thick plate. If it is too thick, it cannot be rolled into a coil. A thickness of more than 16 millimeters cannot be rolled into a coil.

Stainless steel sheets are divided into hot-rolled and cold-rolled according to the rolling process of the rolling mill.

Hot-rolled sheets are usually marked as NO. 1, while cold-rolled sheets are marked as 2B or BA (BA has a better surface than 2B, it is bright and close to a mirror, and the best is a mirror finish, which is not readily available and requires processing).

For example, 304 is often purchased in the form of a sheet, which means that the coil must be flattened into a sheet by a leveling machine.

Domestically produced cold-rolled sheets have a thickness of 3 millimeters or less, while hot-rolled sheets have a thickness of more than 3 millimeters.

Sheets with a thickness of 3 millimeters or less can be both hot-rolled and cold-rolled, but imported cold-rolled sheets can have a thickness of 4 millimeters or even 6 millimeters.

Hot-rolled sheets with a thickness of 3 millimeters to 12 millimeters are called medium plates, while those above 12 millimeters are called thick plates, which can be as thick as more than 120 millimeters and are called hot-rolled medium and thick plates. Cold-rolled sheets are thin.

5. Common Stainless Steel Grades: Characteristics and Applications

There are simpler ways to classify stainless steel, which can be roughly divided into three categories: the 200 series, the 300 series, and the 400 series.

Among them, the 300 series is the most commonly used, while the 200 and 400 series are to some extent substitutes for the 300 series.

Strictly speaking, the 400 series is not called stainless steel, but stainless iron, because it does not contain nickel and can be attracted by a magnet.

The 200 and 300 series contain nickel, so they are not magnetic and cannot be attracted by a magnet. 304 is the most common variety in the 300 series, so the trend of the entire stainless steel price can be judged from the price changes of 304.

The 200 series contains less nickel, the 400 series contains no nickel, and the 300 series contains the most nickel. Therefore, the 300 series is most affected by the price of nickel.

The 300 series can be simply divided into 304, 304L, 316, 316L, 321, 309S, 310S, and so on.

The basis for differentiation is the different content of various metal elements.

Stainless steel with different contents of elements has different characteristics.

The difference between 304 and 304L lies in the difference in carbon (C) content. The “L” indicates low carbon content.

The difference between 316 and 316L is also the same.

Series(ASTM)GBSUSPropertyPurpose
2002011Cr17Mn6Ni5NSUS201It has the characteristics of acid and alkali resistance, high density, bubble-free polishing, and no pinholes. It is a high-quality material used for producing various watch cases, straps, and back covers.It is mainly used for making decorative pipes, industrial pipes, and some shallow-drawn products.
2021Cr18Mn8Ni5NSUS202By replacing some of the nickel with Mn and N, it has achieved good mechanical and corrosion resistance properties. It is a new type of nickel-saving stainless steel. Its room temperature strength is higher than that of 304, and it has good oxidation resistance and mid-temperature strength below 800 degrees Celsius.It is mainly used for making decorative pipes, industrial pipes, and some shallow-drawn products.
220500Cr22Ni5Mo3NSUS2205The range of Cr, Mo, and N elements is relatively narrow, making it easy to achieve phase equilibrium and improve the strength, corrosion resistance, and weldability of the steel. It is often used in materials with high property requirements and those that require welding, such as oil and gas pipelines.It is used for heat exchangers, condensers, and devices that require resistance to seawater, high temperatures, concentrated nitric acid, and other harsh conditions in industries such as petroleum refining, fertilizers, papermaking, petrochemicals, and chemical engineering.
3003041Cr18Ni9(06Cr18Ni10)SUS304It has the widest range of applications and good corrosion resistance, heat resistance, low-temperature strength, and mechanical properties. It has good thermal processing properties such as stamping and bending, and does not undergo hardening phenomenon after heat treatment (non-magnetic, usable temperature range -196℃ to 800℃).It is used in the food industry and household items such as class 1 and 2 tableware, kitchen cabinets, indoor piping, water heaters, boilers, and bathtubs.
304J1 SU530411Adding Cu improves the formability, especially the drawability and resistance to age cracking, and allows for the formation of products with complex shapes. Its corrosion resistance is the same as that of 304 steel.It is used in thermos bottles, kitchen sinks, pots, kettles, steel, insulated lunch boxes, door handles, and textile processing machinery.
304L00Cr19Ni10SUS304LThe corrosion resistance of low-carbon 304 steel is similar to that of 304 steel, but its resistance to intergranular corrosion is excellent after welding or stress relief. It maintains good corrosion resistance without high-temperature treatment and is generally used below 400℃ (non-magnetic, usable temperature range -196℃ to 800℃).It is used in outdoor machinery, building materials, heat-resistant parts, and parts that are difficult to heat-treat in the chemical, coal, and petroleum industries that require high resistance to intergranular corrosion.
3211Cr18Ni9TiSUS321Adding Ti to 304 steel can prevent intergranular corrosion and is suitable for use at temperatures ranging from 430℃ to 900℃.It is used in agriculture, components, nuclear energy industry, aircraft, piping, boilers, and automotive parts.
30031600Cr17Ni12Mo2SUS316Adding Mo to the material improves its corrosion resistance, atmospheric corrosion resistance, and high-temperature strength. It can be used under harsh conditions and has excellent work hardening properties.It is used in equipment for seawater, chemical dyes, papermaking, oxalic acid, and fertilizer production; facilities for photography, food industry, coastal regions; ropes, CD materials.
316L00Cr17Ni14Mo2SU5316As a low-carbon series of the 316 steel type, it has the same characteristics as 316 steel and has excellent resistance to intergranular corrosion.It is used in products that have special requirements for resistance to intergranular corrosion in the application of 316 steel.
309500Cr23Ni13Mo2SUS309sThe 23Cr-13Ni high-alloy steel stainless steel has excellent corrosion resistance and strength, and is suitable for components that work at temperatures up to 1000℃.It is used in exhaust machinery, heat treatment furnaces, and heat exchangers.
3105OCr25Ni20Si2SUS310SDue to its high percentage of chromium and nickel, 310S has excellent oxidation resistance, corrosion resistance, and high-temperature resistance. It has good creep strength and can continue to operate under high temperatures, making it suitable for use in high-temperature environments.It is used in boilers, exhaust machinery, heat treatment furnaces, and heat exchangers.
400409L00Cr12TiSUS409LThe most economical type, with added Ti element, has good high-temperature corrosion resistance and high-temperature strength. It belongs to ferritic stainless steel (chromium steel).It is used in products such as automotive exhaust pipes, heat exchangers, and containers that do not require heat treatment after welding.
41010r135U5410It is the representative steel of martensitic steel. Although it has high strength, it is not suitable for use in harsh corrosive environments. It has good workability and can be hardened by heat treatment (magnetic).It is used in cutting tools, mechanical parts, petroleum refining equipment, bolts, nuts, and pump rods.
41050Cr13SU5410  
400420J12Cr13SUS420J1It has high hardness after quenching and good corrosion resistance (magnetic).It is used in tableware (knives) and turbine blades.
420123Cr13SUS42012It has higher hardness after quenching than 42011 steel (magnetic).It is used in cutting tools, pipe nozzles, valves, and flatware.
4301Cr17SUS430As a representative steel of titanium austenitic stainless steel, it has low thermal expansion, excellent formability, and oxidation resistanceIt is used in heat-resistant utensils, burners, household appliances, type 2 tableware, kitchen sinks, exterior decorative materials, bolts, and nuts.
444  It is used in underwater special purpose plates.

6. Calculation of Stainless Steel:

Theoretical weight calculation of stainless steel plate:

Plate weight = Density × Thickness (mm) × Width (m) × Length (m)

1 mm = 100 microns

Calculation of Stainless Steel Specifications:

Stainless Steel Pipe: (Outer Diameter – Wall Thickness) × Wall Thickness × 0.02491 = kg/m;

Stainless Steel Plate: Thickness × (Width × Length) × Density = kg/square centimeter;

Density:

Different stainless steels have different densities:

400 series density is 7.75;

304, 321, 304N, 201, 202, 304L, 301 density is 7.93;

310S, 309S, 316, 316L density is 7.98.

Surface Treatment of Stainless Steel:

Stainless steel is characterized by unique properties such as high strength, high wear resistance, superior corrosion resistance, and resistance to rust.

As a result, it is widely used in the environmental protection, chemical, mechanical and electrical, home appliance, home decoration, and precision industries.

The development prospects of stainless steel applications are becoming increasingly broad, but the development of stainless steel applications largely depends on the level of development of its surface treatment technology.

Surface Finishing Grades

SurfaceFeaturesSummary of Manufacturing MethodsPurpose
NO.1Silver White MatteHot rolled to the specified thickness, on a rough, matte surface that has been annealed and dephosphorizedUses that do not require surface gloss
NO.2Dsilvery whiteAfter cold rolling, heat treatment and acid washing are carried out, and sometimes the final light rolling is carried out on the rough surface of 2 rollers, which is a matte surface processing2D products are used for applications with low surface requirements, such as general materials and deep drawing materials.
NO.2BGloss stronger than NO.2DAfter NO.2D treatment, the polishing roller is used for the final light cold rolling process to obtain suitable gloss. This is the most commonly used surface processing, which can also be used as the first step of polishingGeneral materials
BABright as a mirrorNo standard, usually bright annealed surface processing with high surface reflectivityBuilding materials, kitchen utensils
NO.3Rough grindingGrind NO.2D and NO.2B materials with 100-200 # abrasive grinding beltsBuilding materials, kitchen utensils

Surface finishing method

The following are commonly used stainless steel surface finishing methods:

Surface natural color whitening treatment

Surface mirror brightness treatment

Surface coloring treatment

1. Surface Natural Color Whitening Treatment:

During the processing of stainless steel, black oxidation skin is generated through the processes of coiling, edging, welding, or surface heat treatment. This hard gray-black oxidation skin is mainly composed of two components, NiCr2O4 and NiF2O4.

In the past, a strong corrosion method using hydrofluoric acid and nitric acid was used to remove it. However, this method is costly, pollutes the environment, is harmful to the human body, has high corrosiveness, and is gradually being phased out.

Oxidation skin treatment methods:

(1) Sandblasting (sphere) method: mainly uses the method of spraying micro-glass beads to remove the black oxidation skin on the surface.

(2) Chemical method: use a pollution-free acid pickling passivation paste and a cleaning solution with non-toxic inorganic additives at room temperature.

Thus, the purpose of whitening treatment of stainless steel natural color is achieved. After processing, it basically looks like a colorless appearance. This method is more suitable for large and complex products.

2. Surface Mirror Brightness Treatment:

Depending on the complexity of the stainless steel product and the user’s requirements, mechanical polishing, chemical polishing, and electrochemical polishing can be used to achieve mirror brightness.

The advantages and disadvantages of these three methods are as follows:

Polishing MethodsAdvantagesDisadvantagesApplicability
Mechanical polishingThe flatness and brightness of the parts are good after processingThe labor intensity is high, the pollution is serious, complex parts cannot be processed, the brightness cannot be uniform, the brightness retention time is not long, dull and rusty.Processing simple parts, small and medium-sized products
Chemical polishingLow investment in processing equipment, can polish complex parts, fast, efficient, and good corrosion resistance.The brightness is poor, there is gas overflow, and ventilation equipment is needed, heating is difficult.Processing small batches of complex parts and small parts with low brightness requirements
Electrochemical polishingThe mirror brightness is long-lasting, the process is stable, pollution is low, cost is low, and corrosion resistance is good.High anti-pollution, large one-time investment in processing equipment, complex parts need fixtures and auxiliary electrodes, cooling facilities are needed for mass production.Mass production, mainly used for high-end products, export products, products with tolerances, with stable processing technology and easy operation.

3. Surface Coloring Treatment:

Stainless steel coloring not only gives stainless steel products various colors and increases the variety of colors, but also improves the wear resistance and corrosion resistance of the products.

There are several methods of coloring stainless steel:

(1) Chemical oxidation coloring method:

It is to form a color film through chemical oxidation in a specific solution, including chromic acid salt method, mixed sodium salt method, sulfuration method, acidic oxidation method, and alkaline oxidation method.

Generally, the “INCO” method is used more frequently, but to ensure that a batch of products have consistent colors, a reference electrode must be used to control it.

(2) Electrochemical coloring method:

It is to form a color film through electrochemical oxidation in a specific solution.

(3) Ion deposition oxide coloring method

It is to put the stainless steel workpiece into a vacuum coating machine for vacuum evaporation coating.

For example, the yellow gold watch case and watch strap are generally gold yellow. This method is suitable for mass production of products.

Because of the large investment and high cost, small batches of products are not cost-effective.

(4) High-temperature oxidation coloring method:

It is to immerse the workpiece in a specific melt salt and maintain a certain process parameter, so that the workpiece forms a certain thickness of oxidation film and presents various colors.

(5) Gas-phase decomposition coloring method:

It is relatively complex and is used less in industry.

In addition, stainless steel can also be subjected to surface treatments such as sandblasting, oil grinding, wire drawing, 8K grinding, short wire weaving, rolling flowers, black titanium, color titanium, rose gold, etc.

The choice of surface treatment method for stainless steel should be based on the product structure, material, and different surface requirements.

Heat Treatment of Stainless Steel

1. Heat Treatment of Ferritic Stainless Steel

Ferritic stainless steel is generally a stable single ferritic structure that does not undergo phase changes during heating and cooling, so heat treatment cannot be used to adjust mechanical properties.

The main purpose is to reduce brittleness and improve intergranular corrosion resistance.

① Sigma Phase Brittleness

Ferritic stainless steel is highly prone to generating sigma phase, which is a rich Cr metal compound that is hard and brittle, especially prone to forming between grains, making the steel brittle and increasing intergranular corrosion sensitivity.

Sigma phase formation is related to composition, besides Cr, Si, Mn, Mo, etc. also promote sigma phase formation; it is also related to the processing process, especially heating in the 540-815℃ interval, staying, which promotes sigma phase formation more.

But sigma phase formation is reversible, re-heating to a temperature above the sigma phase formation temperature will dissolve in the solid solution again.

② 475℃ Brittleness

Ferritic stainless steel heated for a long time in the 400-500℃ interval will show the characteristics of increased strength and decreased ductility, that is, increased brittleness, especially at 475℃, which is most obvious.

This is because, at this temperature, the Cr atoms in the ferrite will rearrange to form rich Cr small areas that co-lattice with the matrix, causing lattice distortion and generating internal stress, making the steel harder and more brittle.

At the same time that the rich Cr areas are formed, there must be poor Cr areas, which have a detrimental effect on corrosion resistance.

When the steel is re-heated to a temperature above 700℃, the distortion and internal stress will be eliminated and the 475℃ brittleness will disappear.

③ High Temperature Brittleness

When heated above 925℃ and rapidly cooled, compounds of Cr, C, N, etc. are precipitated in the grain and grain boundary, causing increased brittleness and intergranular corrosion. This compound can be eliminated by heating to 750-850℃ and then rapidly cooling.

Heat treatment process:

① Annealing

In order to eliminate sigma phase, 475℃ brittleness and high temperature brittleness, annealing can be used, heating at 780-830℃, holding, then air cooling or furnace cooling.

For ultra-pure ferritic stainless steel (C≤0.01%, strictly controlling Si, M, S, P), the annealing heating temperature can be increased.

② Stress Relief Treatment

After welding and cold processing, parts may generate stress. If annealing is not appropriate, heating at 230-300℃, holding, and air cooling can eliminate some internal stress and improve plasticity.

2. Austenitic Stainless Steel Heat Treatment

Austenitic stainless steel contains alloy elements such as Cr, Ni, etc., which result in the reduction of the Ms point to below room temperature (-30 to -70°C).

This ensures the stability of the austenitic structure, so that no phase change occurs during heating and cooling above room temperature.

Therefore, the main purpose of the heat treatment of austenitic stainless steel is not to change the mechanical properties, but to improve the corrosion resistance.

Solution Annealing

Purpose:

① Dissolution and precipitation of alloy carbides in steel

Carbon (C) is one of the alloy elements in steel. In addition to a little strengthening effect, it is detrimental to the corrosion resistance, especially when C forms a carbide with Cr, which has a worse effect and reduces stress.

Therefore, based on the characteristics of C changing with temperature in the austenite, that is, the solubility is large at high temperature and small at low temperature, it is reported that the solubility of C in the austenite is 0.34% at 1200°C, 0.18% at 1000°C, 0.02% at 600°C, and even less at room temperature.

So, heating the steel to high temperature, fully dissolving the C-Cr compound, and then quickly cooling to prevent precipitation, ensures the corrosion resistance of the steel, especially the intergranular corrosion resistance.

② Sigma phase

Austenitic steel will promote the precipitation of sigma phase if it is heated for a long time in the 500-900°C range, or if elements such as Ti, Nb, and Mo are added to the steel.

This will increase the brittleness of the steel and reduce its corrosion resistance.

The method of eliminating the sigma phase is also to dissolve it above its possible precipitation temperature and then cool quickly to prevent precipitation.

Process:

The GB1200 standard recommends a wide range of heating temperatures: 1000-1150°C, usually 1020-1080°C. Considering the specific grade composition, whether it is a casting or plating, within the allowed range, the heating temperature should be adjusted appropriately.

If the heating temperature is low, the C-Cr carbide cannot be fully dissolved, and if the temperature is too high, there is also a problem of grain growth and reduction of corrosion resistance.

Cooling method: It should be cooled rapidly to prevent the precipitation of the carbide. In the standards of China and other countries, “rapid cooling” is specified after solution annealing.

Based on different literature and practical experience, the scale of “fast” can be grasped as follows:

· Water cooling for those with C content ≥0.08%, high Cr content (>22%) and high Ni content, or with C content <0.08% but effective size >3mm;

· Air cooling for those with C content <0.08% and size <3mm;

· Air cooling for those with effective size ≤0.5mm.

Stabilization Heat Treatment

Stabilization heat treatment is only applicable to austenitic stainless steel containing stabilizing elements Ti or Nb, such as 1Cr18Ni9Ti, 0Cr18Ni11Nb, etc.

Purpose:

As previously mentioned, Cr combines with C to form Cr23C6 compounds and precipitates at grain boundaries, causing a decrease in the corrosion resistance of austenitic stainless steel.

Cr is an element that forms carbides and will combine with C whenever it has the opportunity, so the addition of elements Ti and Nb that have a stronger affinity for C than Cr will reduce the chance of C combining with Cr, causing Cr to remain stable in the austenite, thus improving the steel’s corrosion resistance.

The role of stabilization heat treatment is to make Ti and Nb combine with C and keep Cr stable in the austenite.

Process:

Heating temperature: This temperature should be higher than the solution temperature of Cr23C6 (400-825℃) and lower than or slightly higher than the start solution temperature of TiC or NbC (such as TiC solution temperature range of 750-1120℃).

The stabilization heating temperature is usually chosen at 850-930℃, which will fully dissolve Cr23C6 and cause Ti or Nb to combine with the C in it, while Cr remains in the austenite.

Cooling method: Usually air cooling is used, but water cooling or furnace cooling can also be used, depending on the specific conditions of the part.

The cooling rate has little effect on the stabilization effect. From our experimental results, when cooling from a stabilization temperature of 900℃ to 200℃, the cooling rates of 0.9℃/min and 15.6 are basically equivalent in terms of microstructure, hardness, and intergranular corrosion resistance.

Stress Relief Treatment

Purpose:

Parts made of austenitic stainless steel inevitably contain stress, such as processing stress during cold processing and welding stress, etc.

The presence of these stresses will bring adverse effects, such as: impact on dimensional stability: parts with stress will experience stress corrosion cracking in Cl-containing media or in media such as H2S or NaOH, which is a sudden, unexpected local failure and is very harmful.

Therefore, parts made of austenitic stainless steel used under certain working conditions should minimize stress as much as possible, which can be achieved through stress relief methods.

Process:

Parts made of austenitic stainless steel inevitably contain stress, such as processing stress during cold processing and welding stress, etc.

The presence of these stresses will bring adverse effects, such as: impact on dimensional stability: parts with stress will experience stress corrosion cracking in Cl-containing media or in media such as H2S or NaOH, which is a sudden, unexpected local failure and is very harmful.

Therefore, parts made of austenitic stainless steel used under certain working conditions should minimize stress as much as possible, which can be achieved through stress relief methods.

3. Martensitic stainless steel heat treatment

Martensitic stainless steel is distinct from ferritic, austenitic, and duplex stainless steel due to its ability to adjust its mechanical properties over a wide range through heat treatment to meet different use conditions.

Different heat treatment methods also have different effects on corrosion resistance.

 Martensitic stainless steel heat-treated microstructure

Based on chemical composition,

0Cr13, 1Cr13, 1Cr17Ni2 are martensitic plus a small amount of ferrite;

2Cr13, 3Cr13, 2Cr17Ni2 are primarily martensitic;

4Cr13, 9Cr18 have alloy carbides in the martensitic matrix;

0Cr13Ni4Mo, 0Cr13Ni6Mo have residual austenite in the martensitic matrix.

Martensitic stainless steel corrosion resistance and heat treatment

Heat treatment of martensitic stainless steel not only changes mechanical properties, but also has different effects on corrosion resistance.

For example, after quenching, low-temperature tempering has higher corrosion resistance; medium-temperature tempering at 400-550℃ has lower corrosion resistance; and high-temperature tempering at 600-750℃ improves corrosion resistance again.

② Martensitic stainless steel heat treatment process and effects

Annealing

Different annealing methods can be used based on the desired results and effects:

If only hardness reduction, ease of machining, and stress elimination are required, low-temperature annealing (sometimes called incomplete annealing) can be used.

Heating temperature can be 740-780℃, air-cooled or furnace-cooled to ensure hardness of 180-230HB;

If improved forging or casting structure, lower hardness, and guaranteed performance for direct use are required, full annealing can be used.

Generally heated to 870-900℃, furnace-cooled after holding or cooled to below 600℃ at a rate of ≤40℃/h. Hardness can reach 150-180HB;

Isothermal annealing, which can replace full annealing and achieve the same goal.

Heating temperature is 870-900℃, holding after heating and furnace-cooling to 700-740℃ (refer to transformation curve), holding for a longer time (refer to transformation curve), then furnace-cooling to below 550℃.

Hardness can reach 150-180HB. This isothermal annealing is also an effective way to improve the poor structure after forging, and to improve the mechanical toughness after quenching and tempering.

Annealing

The main purpose of annealing martensitic stainless steel is to strengthen it. By heating the steel to a temperature above the critical point and then holding it, the carbides are fully dissolved into the austenite, then cooling it at an appropriate rate, the martensitic structure is obtained after annealing.

Heating temperature selection: The basic principle is to ensure the formation of austenite and to fully dissolve the alloy carbides into the austenite to homogenize it. The heating temperature must not be too low or too high.

The heating temperature for annealing martensitic stainless steel varies widely depending on the reference data and the recommended range.

According to our experience, the temperature can generally be heated in the range of 980-1020°C.

Of course, for special steel grades or special composition control or special requirements, the heating temperature should be lowered or raised accordingly, but the heating principle must not be violated.

Cooling method: Due to the composition characteristics of martensitic stainless steel, the austenite is relatively stable, the C curve moves to the right, and the critical cooling rate is relatively low.

Therefore, oil cooling or air cooling can achieve the effect of annealing martensitic structure. However, for parts that require deep quenching and high mechanical performance, such as impact toughness, oil cooling should be used.

Hardening

After annealing martensitic stainless steel, martensitic structure is obtained, with high hardness, brittleness, and internal stress. It must be subjected to hardening treatment. Martensitic stainless steel is basically used in two hardening temperatures:

Hardening between 180-320°C. The hardening martensitic structure is obtained, with high hardness and strength, but low plasticity and formability, and good corrosion resistance. For example, low-temperature hardening can be used for cutting tools, bearings, wear-resistant parts, etc.

Hardening between 600-750°C. The hardening austenitic structure is obtained, with good comprehensive mechanical properties such as strength, plasticity, toughness, and initial performance.

Depending on the requirements for strength, plasticity, and toughness, the lower or upper limit temperature can be used for hardening. This structure also has good corrosion resistance.

Hardening between 400-600°C is generally not used because during hardening in this temperature range, a large amount of carbonitrides are precipitated from the martensite, causing hardening brittleness and reducing corrosion resistance.

However, springs, such as 3Cr13 and 4Cr13 steel springs, can be hardened at this temperature, and the HRC can reach 40-45, with good elasticity.

The cooling method after hardening can generally be air-cooled, but for steel grades with a tendency to hardening brittleness, such as 1Cr17N2, 2Cr13, 0Cr13Ni4Mo, etc., it is best to oil-cool after hardening.

In addition, it is important to note that hardening must be carried out in a timely manner, and in the summer, it should not exceed 24 hours, and in the winter, it should not exceed 8 hours.

If hardening cannot be carried out in accordance with the process temperature, measures should also be taken to prevent the formation of static cracks.

4. Ferritic-Austenitic Duplex Stainless Steel Heat Treatment

Duplex stainless steel is a relatively new member of the stainless steel family, having developed later but widely recognized and valued for its unique characteristics.

Duplex stainless steel has a composition characterized by high Cr, low Ni, added Mo, and N, and an organizational characteristic that gives it a higher strength and plasticity than austenitic stainless steel and ferritic stainless steel; corrosion resistance equivalent to austenitic stainless steel; and higher anti-pitting, crevice corrosion and stress corrosion resistance than any other stainless steel in cl- media and seawater.

Purpose:

① Eliminate Secondary Austenite

Under high temperature conditions (such as casting or forging), the amount of ferrite increases, and at temperatures above 1300°C, it can become a single-phase ferrite, which is unstable and will age at lower temperatures, resulting in the precipitation of austenite, known as secondary austenite.

This secondary austenite has a lower Cr and N content than normal austenite and may become a source of corrosion, so it should be eliminated through heat treatment.

② Eliminate Cr23C6 type carbides

Duplex steel will precipitate C23C6 at temperatures below 950°C, increasing brittleness and reducing corrosion resistance, so it should be eliminated.

③ Eliminate Cr2N, CrN nitrides

Due to the presence of N in the steel, nitrides can form with Cr, affecting mechanical and corrosion properties, so they should be eliminated.

④ Eliminate intermetallic phases

The compositional characteristics of duplex steel promote the formation of intermetallic phases such as σ and γ, which reduce corrosion resistance and increase brittleness, so they should be eliminated.

Process:

Similar to austenitic steel, it is treated by aging solution treatment, heated to 980-1100°C, then rapidly cooled, usually with water cooling.

5. Precipitation Hardening Stainless Steel Heat Treatment

Precipitation hardening stainless steel is a relatively new development, a type of stainless steel that has been experimentally tested, summarized, and innovated.

Among the early stainless steels, ferritic stainless steel and austenitic stainless steel have good corrosion resistance but cannot adjust their mechanical properties through heat treatment, limiting their use.

Martensitic stainless steel can adjust its mechanical properties over a large range through heat treatment, but its corrosion resistance is poor.

Features:

It has a lower C content (generally ≤0.09%) and a higher Cr content (generally ≥14% or more), along with added elements such as Mo and Cu, giving it high corrosion resistance comparable to austenitic stainless steel.

Through solution annealing and aging treatment, a precipitated hardening phase can be obtained on the martensitic matrix, giving it high strength and the ability to adjust strength, plasticity, and toughness within a certain range by adjusting the aging temperature.

Additionally, the process of solid solution first and then precipitation hardening through aging can reduce processing costs by allowing basic shaping at a lower hardness after solid solution treatment and then strengthening through aging.

Classification:

① Martensitic Type Precipitation Hardening Stainless Steel and its Heat Treatment

The characteristic of martensitic type precipitation hardening stainless steel is that the beginning temperature of the transformation from austenite to martensite, Ms, is above room temperature.

After heating to austenitize and cooling rapidly, a plate-like martensitic matrix is obtained, and after aging, fine Cu precipitates are derived from the plate-like martensite matrix to strengthen.

For example, in the GB1220 standard, a typical brand is: 0Cr17Ni4Cu4Nb (PH17-4)

The composition (%): C≤0.07, Ni: 3-5, Cr: 15.5-17.5, Cu: 35, Nb: 0.15-0.45; Ms point about 120°C; Mz point about 30°C.

Solution Treatment:

The heating temperature is 1020-1060°C, and after insulation, it is water-cooled or oil-cooled, with a plate-like martensitic structure and a hardness of about 320HB.

The heating temperature should not be too high; if it exceeds 1100°C, the ferrite content in the structure will increase, Ms point will decrease, residual austenite will increase, and hardness will decrease, resulting in poor heat treatment results.

Aging Treatment:

Depending on the aging temperature, the diffusivity and particle size of the precipitated phases are different, resulting in different mechanical properties.

The GB1220 standard specifies the performance after aging at different aging temperatures.

 σb (N/mm2)σs (N/mm2)δ(%)Ψ(%)HB
1040℃Solution Treatment    ≤363
480℃×4h≥13101180≥10≥40≥375
550℃×4h≥1060≥1000≥12≥45≥331
580×4h≥1000≥865≥13≥45≥302
620℃×4h≥930≥725≥16≥50≥277

 Semi-austenitic type stainless steel heat treatment

This type of steel has an Mf point that is generally slightly lower than room temperature, so after the solution treatment is cooled to room temperature, an austenitic structure is obtained with low strength.

To improve the strength and hardness of the matrix, it needs to be heated again to 750-950°C, held at temperature.

During this stage, carbides will precipitate from the austenite, reducing its stability, and raising the Mf point above room temperature. Upon cooling, a martensitic structure is obtained.

Some may also undergo additional cold treatment (sub-zero treatment), after which, aging is performed to finally obtain a martensitic matrix with precipitated strengthening in the steel.

For example, in the GB1220 standard, this recommended precipitation stainless steel grade is 0Cr17Ni7Al (PH17-7). Composition (%): C≤0.09, Cu≤0.5, Ni: 6.5-7.5, Cr: 16-18, Al: 0.75-1.5.

Solution + adjustment + aging treatment

· Solution treatment heating temperature 1040°C, cooling with water or oil after holding at temperature to obtain austenite, with a hardness of around 150HB;

· Adjustment treatment temperature 760°C, air-cooled after holding at temperature to cause alloy carbides to precipitate from the austenite, reducing its stability, and raising the Mf point to 50-90°C. After cooling, a plate-like martensite is obtained, with a hardness of around 290HB;

· Further aged at 560°C, Al and compound precipitated, steel is strengthened, with a hardness of around 340HB.

Solution + adjustment + cold treatment + aging

· Solution treatment heating temperature 1040°C, water-cooled to obtain austenitic structure;

· Adjustment treatment temperature 955°C, raising the Mf point, cooling to obtain plate-like martensite;

· Cold treatment -73°C × 8h, reducing residual austenite in the structure to obtain maximum martensite;

· Aging treatment temperature 510-560°C, causing Al to precipitate and harden through strengthening treatment, with a hardness of 336HB.

Solution + cold deformation + aging

· Solution treatment temperature 1040°C, water-cooled to obtain austenitic structure;

· Cold deformation, using the principle of cold working strengthening, causing austenite to transform into martensite at the Md point, with a cold working deformation greater than 30-50%;

· Aging treatment: aging at around 490°C, causing Al to precipitate and harden.

· Reports show that after 57% cold rolling deformation of the solution austenite, the hardness reaches 430HB, σb reaches 1372 N/mm2, and after aging at 490°C, the hardness reaches 485HB, σb reaches 1850 N/mm2.

It can be seen that after proper treatment, the mechanical properties of precipitation-hardened martensitic stainless steel can fully meet those of martensitic stainless steel, while the corrosion resistance is comparable to that of austenitic stainless steel.

It should be noted that although both martensitic stainless steel and precipitation-hardened stainless steel can be strengthened through heat treatment, the strengthening mechanism is different.

Due to the characteristics of precipitation-hardened stainless steel, it has received attention and widespread application.

The impact of alloy elements on the microstructure and performance of stainless steel.

1. The impact of alloy elements on the polarization performance of iron

The type and content of alloy elements directly affect the corrosion resistance of cast steel. The role of alloy elements is first to impact the polarization performance and electrode potential of iron.

Commonly used metals such as Fe, Cr, Ni, Ti and others have unique polarization forms in the anode polarization process.

After the anode pathway, the anode potential increases, and the anode current (corrosion rate) changes accordingly, with almost the same law. The typical form of the polarization curve is shown in the following figure.

As the anode polarization potential increases, the corrosion current does not decrease uniformly, but first increases, then decreases to a minimum, and then maintains this current during a certain potential increase stage, and then the current increases again.

This type of polarization curve is called an anode polarization curve with activation and passivation transitions, and this type of curve is divided into three regions: the activation region (A), the passivation region (B), and the overpassivation region (T).

Polarization plays a significant role in improving the corrosion resistance of all metals. Enhancing anodic polarization or cathodic polarization factors can both improve the corrosion resistance of the metal.

Factors that remove anodic polarization or cathodic polarization will reduce the corrosion resistance of the metal.

Different alloy elements have different effects on the polarization performance of iron. Elements that expand the passive region, i.e., elements that reduce Ecp, P area potential and increase Er point potential, all improve the corrosion resistance of steel.

Any element that enhances the passive performance, i.e., elements that shift Icp, I1 points to the left, will reduce the corrosion current and improve the corrosion resistance.

Elements that increase the Er point potential will reduce the tendency for pitting corrosion, because when the Er point potential is low, local breakdown of the passive film can easily occur when the potential fluctuates near the over-passivated potential, resulting in pitting corrosion.

Among the commonly used alloy elements in steel, Cr can strongly improve the passive performance of pure iron, and can increase the Ecp, Ep, and Er point potential, shift the Icp, I1 points to the left, and Cr is the most effective element for improving the corrosion resistance of iron.

Alloy elements such as Ni, Si, Mo, etc. can also expand the passive region to varying degrees and enhance passive performance. Mo not only enhances the passive performance of iron but also increases the potential of the Er point, thus improving the anti-pitting performance of iron.

2. The effect of iron electrode potential

The electrode potential of metal solid solutions is always lower than that of other compounds, so in the corrosion process, metal solid solutions are always corroded as the anode.

Improving the electrode potential of iron can improve its corrosion resistance. Research has shown that when Cr is added to iron to form a solid solution, the electrode potential of the iron solid solution can be significantly increased, as shown in the following figure.

The improvement of the electrode potential of the material can significantly improve the corrosion resistance of the material.

Due to the good effect of chromium on the passive performance and electrode potential of iron, chromium has become the main alloying element of various stainless steels.

3. Alloy Elements and Their Impact on the Corrosion Resistance of Stainless Steel

(1) Chromium

Chromium is the main element that determines the corrosion resistance of stainless steel. When the chromium content (atomic ratio) reaches 18, 2/8…, the electrode potential of iron increases dramatically, and the corrosiveness also improves.

Chromium is an α-stabilizing element. Chromium oxide is relatively dense and can form a corrosion-resistant protective film.

(2) Carbon and nitrogen

Carbon can strongly stabilize austenite, and its stabilization ability is about 30 times that of Ni. At the same time, it is the main strengthening element in stainless steel. Carbon and chromium can form a series of carbides, seriously affecting the corrosion resistance of stainless steel.

At the same time, carbon can make the machinability and welding properties of stainless steel worse and make ferritic stainless steel brittle. Therefore, the application and drawing of carbon in the production and development of stainless steel is an important task.

The impact of the combination of carbon and chromium on the formation of stainless steel microstructure is shown in the following figure. The figure shows that when the carbon content is low and the chromium content is high, ferritic microstructure will be obtained.

When the carbon content is high and the chromium content is low, martensitic microstructure will be obtained.

In chromium stainless steel, when the chromium content is below 17%, as the carbon content increases, stainless steel with martensitic matrix can be obtained. When the carbon content is low and the chromium content is 13%, ferritic stainless steel can be obtained.

When the chromium content increases from 13% to 27%, due to the increase in chromium content, the ability to stabilize the ferrite increases, and the carbon content in the steel increases accordingly (from 0.05% to 0.2%), but the ferritic matrix can still be maintained.

(3) Nickel

Nickel is one of the three important elements in stainless steel, nickel can improve the corrosion resistance of stainless steel; nickel is also a γ-stabilizing element and the main element for obtaining single-phase austenite and promoting austenite formation in stainless steel.

Nickel can greatly reduce the Ms point and maintain austenite to very low temperatures (-50°C or below) without martensitic transformation.

The increase of nickel content in austenitic steel will reduce the solubility of C and N, thus increasing the tendency for carbon and hydrogen compounds to precipitate out.

As the nickel content increases, the critical carbon content for intergranular corrosion decreases, that is, the steel’s sensitivity to intergranular corrosion increases. Nickel has no significant effect on the pitting and crevice corrosion resistance of austenitic stainless steel.

In addition, nickel can also improve the high-temperature oxidation resistance of austenitic stainless steel.

This is mainly related to the improvement of the composition, structure, and performance of the chrome oxide film by nickel, but the presence of nickel will reduce the steel’s resistance to high-temperature sulfidation.

(4) Manganese

Manganese is a relatively weak austenite-forming element, but it has a strong role in stabilizing the austenitic microstructure. Manganese can partially replace Ni in austenitic stainless steel, 2% Mn is equivalent to 1% Ni.

Manganese can also improve the corrosion resistance of chrome stainless steel in organic acids, formic acid, and acetic acid, and is more effective than nickel. When the Cr content in the steel is greater than 14%, only adding Mn cannot obtain a single austenitic microstructure.

Since Cr content in stainless steel needs to be greater than 17% to have good corrosion resistance, the austenitic stainless steel used in industry that replaces Ni with Mn is mainly Fe-Cr-Mn-Ni-N steel, such as 12Cr18Mn9Ni5N, etc., and the amount of Fe-Cr-Mn-N austenitic stainless steel without nickel is relatively small.

(5) Nitrogen

Nitrogen elements were mainly used in Cr-Mn-N and Cr-Mn-Ni-N austenitic stainless steel in the early days to save Ni elements. In recent years, nitrogen has become an important alloying element in Cr-Ni austenitic stainless steel.

Adding nitrogen in austenitic cast steel can stabilize the austenitic microstructure, improve strength, and enhance corrosion resistance, especially local corrosion resistance, such as resistance to intergranular corrosion, pitting corrosion, and crevice corrosion.

In ordinary low carbon and ultra-low carbon austenitic stainless steel, the resistance to intergranular corrosion can be improved.

This is because nitrogen affects the precipitation process of carbide chrome during sensitization treatment and increases the chromium concentration at the grain boundary.

In high-purity austenitic stainless steel, there is no precipitation of carbide chrome.

At this time, nitrogen has the following functions: first, nitrogen increases the stability of the passivation film and reduces the average corrosion rate; second, although nitrogen-rich steel has the precipitation of nitrogen-containing chrome, the precipitation rate is very slow, and sensitization treatment does not cause intergranular chromium depletion, which has little effect on intergranular corrosion.

Nitrogen has a inhibitory effect on phosphorus segregation at the grain boundary and can improve the steel’s resistance to intergranular corrosion.

Currently, the nitrogen-containing austenitic stainless steel used is mainly for corrosion resistance, while also having high strength: it can be divided into three types: controlled nitrogen, medium nitrogen, and high nitrogen.

Controlled nitrogen is 0.05% to 0.10% N added to ultra-low carbon (C ≤ 0.02% – 0.03%) Cr-Ni austenitic stainless steel to improve the strength of the steel and optimize the intergranular corrosion resistance and stress corrosion resistance of the steel.

Medium nitrogen contains 0.10% to 0.50% N, which is smelted and cast under normal atmospheric pressure conditions. High nitrogen has a nitrogen content of 0.4% or above, which is smelted and cast under increased pressure, mainly used in solid solution or semi-cold worked states, with both high strength and corrosion resistance.

Currently, high nitrogen austenitic steel with a nitrogen content of 0.8% to 1.0% has been put into practical use and has started industrial production.

(6) Titanium, niobium, molybdenum, and rare earth elements

Titanium and niobium are strong carbide-forming elements, which can form carbides with carbon prior to chrome to prevent intergranular corrosion and improve corrosion resistance. The addition of titanium and niobium must maintain a certain ratio with the carbon in the steel.

Molybdenum can improve the passivation ability of stainless steel, expand its passivation media range, such as in hot sulfuric acid, dilute hydrochloric acid, phosphoric acid, and organic acids.

The passivation film containing molybdenum has high stability in many media and is not easily dissolved. It can prevent the damage of Cl to the passivation film, so stainless steel containing molybdenum has the ability to resist pitting corrosion.

Adding rare earth elements such as Ce, La, Y, etc. to stainless steel can solid-solve in the matrix in trace amounts, purify the grain boundary, transform intermetallic compounds, uniform the microstructure, reduce the precipitation of precipitates and segregation at the grain boundary, thereby improving the steel’s corrosion resistance and mechanical properties.

4. The Influence of Alloy Elements on the Microstructure of Stainless Steel

The influence of alloy elements on the microstructure of stainless steel can be divided into two categories: ferritic-forming elements, such as chromium, platinum, silicon, titanium, niobium, etc., and austenitic-forming elements, such as carbon, nitrogen, nickel, manganese, copper, etc.

When these two different elements are added to steel at the same time, the microstructure of stainless steel depends on their combined effects.

For simplicity, the effect of ferritic-forming elements is converted into the effect of chromium, referred to as chromium equivalent [Cr], and the effect of austenitic-forming elements is converted into the effect of nickel, referred to as nickel equivalent [Ni].

The actual composition and obtained microstructure state of the steel can be represented by the diagram made according to the chromium equivalent [Cr] and nickel equivalent [Ni], as shown in the following figure.

As shown in the figure, 12Cr18Ni9 steel is in the A phase area and is an austenitic stainless steel; Cr28 stainless steel is in the ferritic phase area and is a ferritic stainless steel; 30Cr13 stainless steel is in the martensitic phase area and is a martensitic stainless steel.

To achieve single-phase austenitic microstructure, these two types of alloy elements must reach a certain balance. Otherwise, there will be a certain amount of ferritic microstructure in the steel, resulting in a duplex microstructure.

Stainless steel can also rust

The main factors affecting the corrosion of stainless steel are three points:

1. The content of alloy elements.

In general, if the chromium content is 10.5%, the steel is not easily rusted. The higher the content of chromium and nickel, the better the anti-corrosion properties.

For example, 304 materials require a nickel content of 8-10% and a chromium content of 18-20%. Under normal circumstances, such stainless steel will not rust.

2. The smelting process of the manufacturing company also affects the corrosion resistance of stainless steel.

Large stainless steel factories with good smelting technology, advanced equipment, and advanced processes can ensure the control of alloy elements, the removal of impurities, and the control of the cooling temperature of the steel.

Therefore, the product quality is stable and reliable, with good intrinsic quality and not easy to rust.

On the other hand, some small steel factories have outdated equipment and processes, impurities cannot be removed during smelting, and the produced products are bound to rust.

3. External environment, a dry and well-ventilated environment with good climate is not easy to rust.

On the other hand, areas with high air humidity, continuous rainy weather, or air with high acidity and alkalinity are prone to rust. 304 stainless steel, if the surrounding environment is too poor, will also rust.

How to deal with rust spots on stainless steel?

1. Chemical method:

Use an acid wash paste or spray to assist in the re-passivation of the corroded part to form a chromium oxide film, restoring its corrosion resistance. After the acid wash, it is very important to properly rinse with clean water to remove all pollutants and acid residues.

After all treatments, use a polishing machine to polish again and use wax to seal it. For slight rust spots, a mixture of 1:1 gasoline and engine oil can be used with a clean cloth to wipe away the rust spots.

2. Mechanical method:

Sandblasting cleaning, cleaning with glass or ceramic particles, immersion, brushing and polishing. The mechanical method may erase previous pollution, polishing material, or immersion material caused by the removed material. All types of pollution, especially foreign iron particles, can be a source of corrosion, especially in humid environments.

Therefore, surface cleaning with mechanical methods should ideally be carried out under dry conditions. The mechanical method only cleans the surface and does not change the corrosion resistance of the material itself.

Therefore, it is recommended to polish again with a polishing machine and seal with wax after mechanical cleaning.

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