Martensitic stainless steel and austenitic Stainless Steel

stainless steel tube1.Austenitic Stainless SteelIt contains more than 18% of chromium, also contains about 8% of nickel and a small amount of molybdenum, titanium, nitrogen and other elements. Which has good overall performance , corrosion resistant variety of media and its normal grades are: 1Cr18Ni90Cr19Ni9 and others. 0Cr19Ni9 contains C<0.08% which contains amount of Ni and Cr.The steel was austenitic state at room temperature. Such steel has good ductility, toughness, weld ability and corrosion resistance which has good corrosion resistance in both oxidizing and reducing media. So they are used to make equipments anti nitric acid such as anti corrosion containers and equipment lining, transport pipelines, parts of anti nitric acid corrosion. Austenitic stainless steel commonly treated by solution treatment that is heated to the upcoming 1050 ~ 1150 , then cooled to obtain single- phase austenite.

 

2. Duplex stainless steel: It has the advantages of both austenitic and ferritic stainless steel, and has super plasticity. Austenitic and ferritic stainless steel is of about half. In the case of containing a low C, Cr content of 18% ~ 28%, Ni content of 3% to 10%. Some steel further contains Mo, Cu, Si, Nb, Ti, N and other alloying elements. It has both austenitic l and ferritic stainless steel features. Compared with ferrite, it has higher ductility and toughness, no brittle at room temperature. Resistance to intergranular corrosion and welding properties are significantly improved. Meantime it maintains 475 brittleness of ferritic stainless steel and high thermal conductivity, with super plastic characteristics.Compared with the austenitic stainless steel, it has high strength and has significantly improvement in resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent resistance to pitting corrosion and a nickel saving stainless steel.

 

3. Martensitic stainless steel: Martensitic stainless steel has high strength but ductility and weld ability are poor. Martensitic stainless steel normal grades are:420、430、410、409 and others.Due to higher carbon contents, Martensitic stainless steel has a high strength, hardness and wear resistance, but somewhat less corrosion resistance. Martensitic stainless steel are normally used for some parts which require higher mechanical performance and general corrosion requirements, such as springs, turbine blades, valves of hydraulic machines. Such steel is used after quenching, tempering.

Difficulties and Countermeasures of automatic lathe turning of stainless steel 420

CNC1 Analysis of problems and reasons in the cutting process

 

In the part of trial production we took turning test of stainless steel 420 stainless steel according to the process of turning ordinary carbon steel. The tool wear is very serious, productivity is extremely low and the surface quality of parts is not up to the requirements.

Comparison of mechanical properties of stainless steel 420 steel with 40# steel, 45# steel of carbon structural steel, stainless steel 420 steel strength is higher than 40# steel and 45# steel, it is a kind of high strength, good plasticity of medium carbon martensitic stainless steel. Because of serious work hardening, great cutting force, high cutting temperature, all lead to serious tool wear, knife replace increase and downtime and machine adjustment time increase, finally productivity reduced. At the same time, tools is easy to stick and generate steel scrap bue which caused the change of size and surface roughness. And the chip is not easy curling and easy breaking which damage the machined surface and affect part quality. So we can not use the process of cutting 45 steel, nor the general lathe machining method used. Because the general automatic lathe has less tool which demand the best to have one pass making the surface to be machined to meet the requirements of the size and the surface roughness to ensure high productivity.

2 main technical measures

1) Change the hardness of the material by heat treatment

Martensitic stainless steel has different hardness after heat treatment which has great impact on the turning processing. Table 1 is the turning result of stainless steel 420 steel with different hardness after heat treatment machined by tool YW2 materials. Obviously annealed martensitic stainless steel like 0.10.10.1 state has poor turning performance even if the hardness is low. This is because the material has great plasticity and toughness, inhomogeneous microstructure, adhesion, strong weld property, easy to produce scrap bue which all not easy to get a good surface quality. However after quenching and tempering treatment stainless steel 420 material with hardness below HRC30 can has good machining performance and can be easy to reach the better surface quality. Use the material with hardness above HRC30 also can has good quality but the tool easy to be wear. So in the conditions allow the material hardness should be HRC25 HRC30 by quenching and tempering treatment before machining.

Table 1

2) Tool material selection

Commonly In the automatic lathe for turning stainless steel the hard alloy tool materials used are: YG6, YG8, YT15, YT30, YW1, YW2 and others.

High speed steel tool materials commonly used are: W18Cr4V, W6M05Cr4V2AL and others. We had the turning test for tools of several materials with the same conditions of cutting parameters. We can see from table 2: the cylindrical turning tool with TiC-TiCN-TiN composite coating blade has high durability, high cutting speed is desirable, high surface quality and high productivity. However, we have no cutting blade of this material. Through the contrast test of table 2, the cutting performance of YW2 cemented carbide is good and it is used for cutter blade.

Table 2 Different tool cutting contrast test

Material   The geometric angle of tool   Cutting consumption   Tool durability

Min   Surface roughnessμ

Rake angle γ  Back angle α    main angle Kr     edge angle λ

m/min   s

mm/r

YG8 15 ° 8 ° 90 ° -10 45 ~ 550.172 ~ 82 Ra3.2

YT14 15 ° 8 ° 90 ° -10 45 ~ 550.180 ~ 95 Ra3.2

YW2 15 ° 8 ° 90 ° -10 45 ~ 550.190 ~ 110 Ra3.2

Tic-TiCN-TiN

Composite coating blade 12 ° 8 ° 90 ° -4 45 ~ 550.1128 ~ 185 Ra6.3 (off)

Ra Ra1.6

 

3) Selection of the tool geometry and structure

For good tool materials it is very important to choose reasonable geometric angle.

1) Rake angle: For martensitic stainless steel the turning tool rake angle 10 °20 °is suitable.

2): Back angle  5 ° 8 ° is more appropriate, but 10 degrees maximum.

3) Cutting edge inclination angle λ:  selecting  -10 ° 30 °.

4) The main deflection angle of Kr should be selected according to the shape of parts, machining position and tool condition.

5) The edge surface roughness should be no more than Ra0.4 Ra0.2.

In structure, our foreign circular tool using external oblique arc chip breaker groove, on the tool tip chip curling radius is big, on the outer edge of chip curling radius is small, the chip will be turned to the surface to be machined and broken. The cutting knife, we will associate the angle control within 1o, this can improve chip removal condition, prolong the service life of the cutter.

1) Selection of the cutting

The cutting theory is that the cutting speed of V has the greatest impact on cutting temperature and tool wear. The second is cutting amount s and the third is cutting depth t. For machining surface in the automatic lathe the cutting depth of t is decided by parts size and blank size. Generally it is 0 3mm. The cutting speed of hard processing material is much lower than ordinary steel cutting speed.

Cutting feed s has low influence to the durability of the tool than the cutting speed. But it will affect the chip removal and breaking, pull or scratch the surface of the parts, affects the surface quality. Generally believed that if the depth is large, the cutting feed should be reduced and the cutting feed is smaller than outside the circle. For the processed surface roughness requirements is not high the s can be 0.1mm/r 0.2mm/r.

In short, for difficult materials to machine general selection is of slower cutting speed and medium cutting feed.

 

 

2) The appropriate cooling and lubricating liquid

Cooling and lubricating liquid for cutting stainless steel should have the following characteristics:

A. The high cooling performance which can take a large amount of cutting heat.

B. Stainless steel is toughness, easy to produce the knife cutting tumor, deterioration of machining surface. So it requires cooling and lubricating liquid has high performance of lubrication.

C. Good permeability which can play a better role of wedging, diffusion and internal lubrication for that not easy to chip away.

The commonly used cooling and lubricating liquid for stainless steel are as follows: the vulcanized oil, sulfide oil, acid kerosene oil or plant oil, carbon tetrachloride with mineral oil, emulsion etc..

Considering the sulfur corrosion effects on the machine, vegetable oil (such as oil) easily attached to scab in machine tools and deteriorating, we choose with oil according to the weight ratio of 1:9. The permeability of carbon tetrachloride is good and oil has good lubricating ability. Test shows that this kind of lubricant oil is suitable for the small surface roughness requirements of stainless steel parts of semi finish machining and finish machining process, particularly suitable for martensitic stainless steel parts.

3 Conclusion

Due to adopting a series of measures the stainless steel 420 machining process has been completely in the normal state. Sharpening frequency has been 1/3 of the trial production period. The production efficiency improved and the product quality fully meets the design requirements.

Characteristics of X20cr13|420 stainless steel pipe

special alloy tube

Standard: AISI420UNS42000, DIN 17440

X20Cr13|420 stainless steel features and scope of application: High hardness at quenching condition, good corrosion resistance. The density of X20Cr13|420 stainless steel is 7.75g/cm3. The quenching hardness of X2Cr13|420 stainless steel: In general X2Cr13|420 stainless steel quenching temperature at 980 1050 . The hardness at 980 oil quenching is obviously lower than 1050 oil quenching. The hardness at 980 oil quenching is 4550HRC. The hardness at 1050 oil quenching is higher than 2HRC. However, the grain is coarse and brittleness. Recommend the use of 1000 quenching hardness can be obtained and good organization.

STAINLESS STEEL PIPE
420 stainless steel

 

  X20cr13|420 stainless steel needs to preheat before welding. It’s no need preheating for welding Q235 with austenitic stainless steel electrode such as A302 and A307. There should control dilution rate, not swing and smaller current used while welding.

 

Welding consumables can be selected as following:

1. Atmospheric corrosion resistance: G202 and G207preheat 150-300 , 700-730 tempering after welding.

2. Organic acid resistant and heat-resistant: G211, preheat 150-300 .

3. Good plasticity: A102, A107, A202, A207, A302 and A307, preheating for repair welding or thick bulky.

 

Chemical composition

C 0.160.25

Si1.00

Mn1.00

S 0.030

P 0.035

Cr12.0014.00

Niallowed contain 0.60