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3Cr13 stainless steel spring vacuum high pressure gas quenching heat treatment

1 Introduction

3Cr13 stainless steel material, because it contains more alloying element chromium, the C isothermal transformation curve of the material is shifted to the right, and the critical cooling rate is small, which makes the material after austenitization can also form martensite in air-cooled conditions , so it can realize vacuum high pressure air cooling heat treatment. The heat treatment of the spring made of this material originally used salt furnace quenching, and the phenomenon of material pitting corrosion and surface red ash could not be avoided during salt furnace quenching. The corrosion phenomenon on the surface obviously improves the surface quality of the spring. But sometimes the hardness is difficult to control, the quenching microstructure is not ideal, the grains are relatively coarse, and the carbides are distributed in chains. The vacuum air quenching heat treatment process of 3Cr13 must be fully studied, combined with the comprehensive mechanical properties and microstructure characteristics of the material, to determine reasonable vacuum heat treatment parameters, to avoid the appearance of coarse superheated structure and to prevent the tempering brittleness tendency during the heat treatment process, ensure product quality.

2 Process test

Since 3Cr13 has relatively high carbon content in Cr13 type stainless steel, and there are many carbides, in order to fully dissolve these refractory carbides into the matrix during heating, a slightly higher quenching heating temperature was selected. The austenite in the test The melting temperature was in the range of 1025°C to 1060°C. In addition, during quenching and heating, the material has an α-y phase transition, and the thermal conductivity of the material is poor. The volume stress generated by the phase transition during the heating process and the thermal stress generated by different parts of the material are relatively large. In order to prevent deformation and Cracking, the preheating method was used in the heating process. Using the RVGQ vacuum gas quenching furnace, the following test scheme was formed:

(1) According to the technical specification of a spring, the spring sample and tensile sample are processed, and the material is 3Cr13ψ4.6 steel wire.

(2) Group the tensile sample and the spring sample, and perform vacuum high pressure gas quenching treatment, partial pressure gas: argon or nitrogen, cooling gas: nitrogen, air cooling pressure: 5±1bar.

(3) After quenching, compare the metallographic chamber structure of each group of spring samples.

(4) Measure the tensile strength, elongation, and yield strength values of each group of tensile specimens.

3 Analysis and discussion

3.1 Determination of quenching process

The process characteristic of vacuum heat treatment is that it relies on heat radiation heating. The heating body heats up quickly with almost no heat loss, while the workpiece heats up slowly, that is, there is a heating lag time. In order to reduce the exposure time at the quenching holding temperature, preheating should be used both from the perspective of part deformation and from the point of view of obtaining fine particles. Since the quenching temperature of 3Cr13 is 1020~1050℃, generally 650℃ or 800~850℃ should be used for two preheating. Considering the small size of the spring parts, 800~-850℃ can be used for one preheating, that is, when the temperature rises to 800℃ ℃, hold for 20 minutes, make the furnace temperature and spring temperature become the same, and then heat up to the austenitizing temperature. At the same time, when calculating the holding time of the austenitizing temperature, the lag time must be added. For a certain spring, the holding time of the austenitizing temperature is more than 30 minutes.

3.2 Determination of tempering process

For the heat treatment of the spring, the most important thing is to obtain a higher elastic limit and fatigue strength, which depends on the strength and toughness of the material after heat treatment. It therefore depends on the microstructure of the material. Because 3Cr13 stainless steel contains a large amount of chromium, the tempering resistance is relatively high. In order to obtain a stable tempering state, the tempering time should be appropriately lengthened. In the test, the tempering time was extended to 3 hours, and good results were obtained. For parts with high precision requirements such as springs, in order not to affect the dimensional stability due to the internal stress of quenching and rapid cooling, after quenching, a low-temperature tempering process to relieve stress is added.

3.3 Working vacuum degree of heating chamber

The purification and degassing of the material by the working vacuum degree of the vacuum furnace can improve the performance of the material to a certain extent. The ideal vacuum heat treatment process for 3Cr13 stainless steel springs is:

Vacuum degree when heating parts in furnace: <10-2mbar, heating rate 10℃/min; preheating: 800±10℃, 20 minutes; heating up to 1020±5℃, backfilling nitrogen with 0.04-0.1mbar for partial pressure , heat preservation for 50 minutes, nitrogen cooling, air cooling pressure 5bar:

During vacuum tempering, the temperature was raised to 470±10°C at a rate of 10°C/min, kept for 2 hours, vacuum degree <10-2mbar, nitrogen cooling, air cooling pressure 2bar.

After heat treatment through this process, the surface quality of the spring is greatly improved, no corrosion phenomenon occurs, and the geometric size and elastic force are stable, which meets the process requirements. At the same time, its microstructure and comprehensive mechanical properties are better.

4 Application prospects

Vacuum heat treatment technology is a high-precision, high-quality, energy-saving, clean and pollution-free processing and manufacturing technology in material modification. Compared with other heat treatment methods, it has the following outstanding advantages:

(1) To prevent surface oxidation and decarburization:

(2) Vacuum degassing improves the surface purity of the material, improves the fatigue strength, plasticity and toughness of the material, and improves corrosion resistance;

(3) The stability and repeatability of the vacuum heat treatment process are good;

(4) The quenching deformation is small. Because vacuum heating is high temperature radiation heating, the workpiece heats up slowly, the temperature difference between the surface and the core is small, and the thermal stress is small, so the deformation of the vacuum heat treated workpiece is 1/2-1/10 of that of salt bath quenching;

(5) Low energy consumption, safe operation, high degree of automation, good working environment, no pollution and no pollution.

Through the research on vacuum heat treatment of 3Cr13 material, we have a more comprehensive understanding of the characteristics of vacuum high pressure gas quenching heat treatment, which is helpful for us to analyze other vacuum heat treatment process problems such as vacuum oil quenching, vacuum tempering treatment, vacuum carburizing heat treatment The question provides a good reference. We have used the vacuum heat treatment process for the following materials:

(1) Vacuum high pressure gas quenching: martensitic stainless steel ANSI410, 440C, 9Cr18, precipitation hardening stainless steel 15-5PH, 17-4PH, etc., ultra-high strength steel S155 (300M), S99, etc.

(2) Vacuum oil quenching and tempering: 3Cr13, GH4169, 50CrVA, 65Si2MnVA, etc.

Selection of vacuum heat treatment equipment:The vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment of tools and molds. The good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum tempering furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.