Vacuum Heat Treatment of Stainless Steel

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First look at the following typical materials and parts that require vacuum heat treatment. Austenitic stainless steel is mostly made of sheet metal parts with complex shapes, which require multiple stamping and stretching forming. Annealing treatment is required before and during the forming process. The purpose is to soften the material and eliminate the effect of cold work hardening to restore its plasticity. At the same time, in the stamping or drawing process, in order to prevent scratches on the parts, it is necessary to apply grease or other lubricants on the surface of the parts. Practice has proved that even if the use of high-efficiency water-soluble cleaning agents, gasoline, alcohol, acetone, etc., it is difficult to thoroughly Remove this dirt. When austenitic steel is quenched in an ordinary air furnace (solution treatment), the grease and hydrocarbons that have not been cleaned will be oxidized and decomposed into carbon atoms and absorbed by the hot steel, and the internally contaminated surface is slightly contaminated. After quenching in water, these parts show mottling, and the entire part cannot get a uniform oxide layer. After the oxide scale is removed and pickled, the corrosion resistance of these parts (carburized parts) is reduced and corroded It becomes a small pit, causing serious scrapping.

Satisfactory results can be obtained by vacuum heat treatment, especially the high vapor pressure of oil and fat hydrocarbons, which are evaporated and removed by the vacuum pump shortly after vacuuming and heating, so it is impossible to produce local carburization, and at the same time It is guaranteed to meet the plasticity requirements of continued molding, and has no effect on the intergranular corrosion performance of the material.

The valve housing and hydraulic coupling parts made of 2Cr13, 3Cr13, and martensitic stainless steel have very small diameter oil and gas passage holes, internal threads, and blind holes, and the valve seat has a comparison of dimensional accuracy and smoothness requirements. High, it is absolutely not allowed to produce oxidation and corrosion points during heat treatment, otherwise the sealing will not be good, which will affect the normal operation of the valve. After vacuum heat treatment, a smooth surface can be ensured, without oxidation and corrosion points, and the distortion after heat treatment is small. Ideal product technical indicators.

Austenitic, martensitic stainless steel and precipitation hardening stainless steel precision casting blades and various other parts, their surface finish and dimensional accuracy have reached the technical requirements before heat treatment, and generally do not undergo mechanical processing or seldom processing. For martensitic stainless steel and precipitation hardening stainless steel, etc., in order to improve the strength and ductility, after casting, it is necessary to perform homogenization treatment at about 1140°C to make the free ferrite spheroidize to a certain degree, followed by quenching and tempering. Or solution treatment and so on.

These requirements require high surface and dimensional accuracy, and after several heat treatments, it is required to prevent oxidation and depletion of surface alloying elements during heat treatment of parts. The depth of the depleted layer of oxidation and alloying elements on the heat-treated surface must not exceed 0. 0254mm, to minimize the distortion after heat treatment, vacuum heat treatment is the most suitable.

Table 1 shows the vacuum heat treatment process specifications of commonly used stainless heat-resistant steels. In addition to quenching and tempering and solution treatment, homogenization annealing or stress relief annealing can also be carried out in vacuum.

Alloy grade	Quenching			Temper			hardness /HRC
	Tempera ture/℃	Vacuum* 133Pa	cool down	Tempera ture/℃	Vacuum* 133Pa	cool down
2Cr13	1040~1060	10-2	N2 gas or oil	230士30	760	Air cooling	40~40
				250士30			36~40
				560士20	10-2~10-3 or 380~600	Furnace cold or N2 gas	32~36
				580士20			–
4Cr13	1210~1240			250士30	760	Air cooling	40~50
9Cr18	1220~1250	<10-2		200士30			50~60
Cr17Ni2	950~1040	10-2	oil	500土20	10-3	N2 gas	45~50
				250士30	Air furnace	Air cooling
1Cr18Ni9Ti	1100~1500	10-2~10-3	N2 or Ar gas	–	–	–	–
1Cr21Ni5Ti	950~1050
1Cr11Nil2 W2MoV	1000~1020		N2 or oil	660~ 710	10-3	N2 gas	HB3.4~3.7
				540~ 600	10-2~10-3		3.1~3.45
1Cr14Ni13 W2VB	1050士10			660~ 680	10-2~10-3	N2 gas	HB3.3~3.6 3.1~3.35
				550~ 600
				aging
				480 495 550 580	10-2~10-3 or Ar 380~600	Inert gas         Ar gas	>40 >38 >35 >31
0Cr17Ni4 Cu4Nb	1030~1050	10-2	Ar gas Or oil
0Cr17Ni7AI (17-7PH)   0Cr115Ni17 Mo2AIC 15- 7MoPH)	Solid solution 1050 ±10+ adjustments 760 ± 10	10-2	Ar gas   —	aging	High purity Ar		≥HB363
				565士10     aging 510士10	380~600 or 10-3
	Solid solution 1050 ±10 Ten adjustment 950 ± 10-73 ℃ cold treatment	10-2~10-3 10-2 –					≥HB383

Table 1 Vacuum heat treatment process specification of commonly used stainless heat-resistant steel

Note:

1. When the quenching temperature is the same, the grain size of vacuum quenched martensitic stainless steel is one level larger than that of ordinary electric furnace, which has no effect on the impact toughness of the material.

2. The cooling rate of duplex stainless steel (1Cr21Ni5Ti) should not be too slow. There will be brittle phase precipitation at 700~800℃, and brittleness at 475℃ at 500~650℃, so the cooling speed is very important.

3. Martensitic stainless steel can be oil quenched or gas quenched.

4. The austenitic nickel-chromium steel is cooled with argon and partial pressure to prevent micro-N from deteriorating mechanical properties.

5. For thin parts, use one-stage or two-stage preheating.

In the formulation and production process of the vacuum heat treatment process, there are several key issues that need to be paid attention to, which will directly affect the product quality and production efficiency.

(1) When the heat treatment and quenching temperature are the same, the grain size of vacuum-quenched martensitic stainless steel is one level larger than that of ordinary electric furnace heating and quenching. This is the effect of vacuum degassing which hinders the growth of grains in the steel. Gas impurities are related to the removal of gas compounds, but have no significant effect on the impact toughness of the material.

(2) The austenite stability of martensitic stainless steel is not too high, and oil cooling is usually required to achieve complete quenching. When the section of the martensitic stainless steel part is not too thick, it can be hardened by vacuum quenching, the surface brightness is good, and the distortion is small.

(3) When formulating the vacuum heat treatment process specifications for stainless steel parts, it should be determined according to the material, shape, thickness of the parts, the amount of distortion after heat treatment, and dimensional accuracy. For parts with no distortion requirements or thin wall thickness after heat treatment, preheating can be eliminated, and the temperature can be directly raised to the quenching (solution) temperature. Commonly used martensitic and precipitation hardening stainless steels have a temperature range of 250°C for tempering and aging. The following low temperature tempering and 500~700°C high temperature tempering. Low temperature tempering can be carried out in an ordinary air resistance furnace or other furnaces, high temperature tempering can be carried out in a vacuum furnace, and the tempering time can be calculated as 1.5 times the time in Table 2.

Diameter or thickness/mm	Tempering time/min
≤10	30~ 60
10~ 30	60~ 75
31~45	75~ 90
46~77	90~ 120
78~ 100	120~ 150

Table 2 Vacuum tempering time with inert gas