Classification And Relative Machinability of Stainless Steel

What is stainless steel?

Generally, more Cr, Ni, Mo, Ti and other elements are added to the steel to make it have wear resistance. The alloy steel with higher strength at higher temperature (>450 ℃) is called stainless steel.

Generally, Cr content in stainless steel is more than 10%, or Ni content is more than 8%.

Stainless steel is widely used in aviation, aerospace, power generation equipment manufacturing, construction, food and other industrial sectors and daily life.

1. Classification of stainless steel

The classification and characteristics of stainless steel are shown below:

Martensitic stainless steels

The matrix structure is martensite, which can be strengthened by heat treatment and has high strength. It is suitable for manufacturing parts that work in weak corrosive medium and require high strength and wear resistance.

Martensitic stainless steel has high plasticity and toughness in annealed state, so it is difficult to cut;After quenching, the machinability mainly depends on the hardness.

Those with hardness above 38hrc are difficult to process.

Typical martensitic stainless steels include 1Crl3, 2Cr13, 3Cr13, 4Cr13, 9Cr18, etc.

Ferritic Stainless Steel

The matrix structure is ferrite, which is a corrosion-resistant stainless steel at room temperature.

It is characterized by low strength and hardness, good plasticity and poor heat resistance, and its mechanical properties cannot be changed by heat treatment.

Ferritic stainless steel is a kind of stainless steel which is easy to process.

When the C content is 16%~18%, the machinability is good, which is similar to martensitic stainless steel with medium hardness;

However, when the Cr content increases to 25%~30%, it is difficult to process.

Typical ferritic stainless steels include 0Cr13,1Cr17Ti,1Cr28,1Cr17Mo2Ti, etc.

Austenitic stainless steel

The matrix structure is austenite.In addition to Cr content (0% – 18%), it also contains a large amount of Ni (8%~25%), which is a non-magnetic chromium nickel steel.

Austenitic stainless steel can not be strengthened by heat treatment, but has a large tendency of cold work hardening.

It is characterized by high-temperature corrosion resistance, high-temperature strength, plasticity and toughness.

Therefore, it is widely used.The machinability of this kind of stainless steel is much worse than the first two kinds.

Typical steel grades include 0Crl8Ni9, 1Cr18Ni9, 2Cr18Ni9, 1Cr18Ni9Ti, etc.

Austenitic + ferritic stainless steel

This kind of stainless steel is similar to austenitic stainless steel, but it contains a certain amount of ferrite in the structure.

Austenitic + ferritic stainless steel is difficult to deform, but it has the advantage of being insensitive to intergranular corrosion, and has the tendency of dispersion strengthening (that is, after a certain heat treatment, hard points such as carbides and nitrides with very small particles will be precipitated from the grains), thus improving the mechanical properties.

This kind of stainless steel is more difficult to cut than austenitic stainless steel.

Typical steel types include 1Cr21Ni5Ti, 1Cr18Mn10Ni5Mo3Ni, 0Cr17Mn13Mo2Ni, etc.

2. Relative machinability of stainless steel

The relative machinability of stainless steel is shown in table 7-8.

Table 7-8 relative machinability kV of stainless steel

Type

austenitic stainless steel

Martensitic stainless steels

Ferritic Stainless Steel

Austenitic + ferritic stainless steel

Hardness(HRC)

Wa(%)

<28

28~35

>35

16~18

25~30

Relative machinability coefficient Kv

0.5~0.7

0.9~1.1

0.8~0.9

0.5~0.6

0.8~0.9

0.6~0.8

0.4~0.6

Causes and measures of stainless steel cutting difficulty

Stainless steel is widely used in aviation, aerospace, power generation equipment manufacturing, construction, food and other industrial sectors and daily life.

3. Reasons why stainless steel is difficult to cut

The reasons why stainless steel is difficult to cut are shown in table 7-9.

Table 7-9 causes for difficult cutting of stainless steel

ReasonInstruction
High temperature strength, high work hardening tendencyCompared with one strand steel, the strength and hardness of stainless steel are not high, but because it contains a large number of Cr, Ni, Mn and other elements, it has good plasticity and toughness, high high temperature strength and high work hardening tendency. Therefore, the cutting load is heavy.In addition, during the cutting process of austenitic stainless steel, some carbides will precipitate inside, which will aggravate the scratch effect on the tool.
Large cutting forceStainless steel has large plastic deformation during cutting, especially austenitic stainless steel (its elongation is more than 1.5 times that of 45 steel), which increases the cutting force.
The chip and tool are seriously bondedDuring the cutting process, it is easy to generate bue, which not only affects the machined surface roughness, but also easily causes the tool surface to peel off.
Chips are not easy to curl and breakFor closed and semi closed chip containing tools, it is easy to produce chip blockage, which increases the machined surface roughness and causes tool edge collapse.
Large linear expansion coefficientIt is about one and a half times of the linear expansion coefficient of carbon steel. Under the action of cutting temperature, the workpiece is easy to produce thermal deformation and affect the dimensional accuracy.
Low thermal conductivityGenerally, it is about 1/412 of the thermal conductivity of medium carbon steel, with high cutting temperature and fast tool wear.

4. Specific measures for cutting stainless steel

The specific measures for cutting stainless steel are shown in table 7-10.

Table 7-10 specific measures for cutting stainless steel

MeasureInstruction
Selection of tool materials1. Cutting tool materials with high hardness, good toughness and heat resistance and low chemical affinity with stainless steel shall be selected;

2. When high-speed steel is used, high-performance high-speed steel such as W2Mo9Cr4VCo8, W6Mo5Cr4V2A1 and W10Mo4Cr4V3A1 should be selected;

3. When cemented carbide is used, YT alloy (class P of ISO) should not be used, and YW or YG tungsten cobalt alloy (class m and K of ISO) containing Ta (Nb) is preferred.

4. Coated cemented carbide can be CA15, CA25, YBM151, YBM251, YBM351, YBG20, YBG252, YBG302, CN251, YB425, ZC05, ZC07, ZM10, etc.

5. YNG151 can be used for cermet and YNG151C can be used for coated cermet.
Selection of tool geometric parameters1. The front angle and rear angle shall be large to reduce cutting deformation and friction with the rear tool surface. γ0= 15°-25°,a。= 8°~10°can be used for carbide turning tool;

2. Negative chamfer should not be too wide, generally by1= (0.5~1.0) /f;

3. Adopt full circle arc chip breaking groove to increase the front angle and achieve good chip breaking effect;

4. To strengthen the strength of the cutter head, enterγ s=-2 ° ~-6 °; In case of intermittent cutting, γs=-5 ° ~-15°;

5. For tools with closed chip holding capacity, the chip holding space shall be appropriately increased;

6. The tool wear standard should be made smaller, which can be about 1/2 of that for general materials;

7. During grinding, ensure that the front and rear cutting surfaces have small surface roughness values.
Selection of Cutting Parameter1. The cutting speed should not be too high to reduce the cutting temperature;

2. The feed rate should not be too large to avoid excessive cutting load, but not too small to avoid the cutting edge working in the cold hard layer formed by the last feed.

Generally, f=0.12~0.18mm/r can be selected for turning austenitic stainless steel, and f=0.07~0.18mm/r can be selected for turning martensitic or ferritic stainless steel;

3. The back cutting amount shall be as large as possible to avoid the rough skin or the work hardening layer of the previous process.
Proper heat treatment of workpieceMartensitic stainless steel can be quenched and tempered, and the hardness shall be controlled between 28~35HRC after quenching and tempering;

Austenitic stainless steel, solution treated.
Selection of cutting fluidCutting fluid with good anti adhesion and cooling performance shall be selected, such as emulsion of extreme pressure additives such as sulfur and chlorine.

Sulfur oil and carbon tetrachloride, kerosene and oleic acid mixed cutting fluid.The liquid supply must be sufficient, and it is best to adopt spray cooling, high-pressure cooling, cold air cooling and other efficient cooling methods.
OtherSelect machine tools with good rigidity and high power.

It is also necessary to improve the rigidity of the tool and workpiece as much as possible, such as increasing the cross-sectional area of the cutter bar and reducing the overhang length.

When selecting cemented carbide bits for stainless steel drilling, it is better to select K30 brand.

When selecting high-speed steel bits, the drill center thickness should be increased compared with the standard type.

At the same time, the horizontal edge should be polished (shortened to about 1/3 of the ungrounded). The drilling edge can be group drilling.
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