What Is Thermal Treatment?

Heat treatment refers to a metal thermal processing process in which the material is in a solid state by means of heating, holding and cooling to obtain the desired structure and properties. In the process from the Stone Age to the Bronze Age and Iron Age, the role of heat treatment is gradually recognized.

In the sixth century BC, steel weapons were gradually adopted. In order to increase the hardness of steel,

Current national standards for heat treatment

1 GB / T7232-2012 metal heat treatment process term 2013-03-01 implemented, replace GB / T 7232-1999

2 GB / T8121-2002 heat treatment process material term 2002-12-01 implemented, instead of GB / T 8121-1987

3 GB / T9452-2003 Method for determining the effective heating zone of a heat treatment furnace 2004-06-01 implemented instead of GB / T 9452-1988

4 GB / T17031.1-1997 Dry heat effect of textile fabrics under low pressure Part 1: Dry heat treatment procedures for fabrics

5 GB / T7631.14-1998 Classification of lubricants and related products (type L) Part 14: Group U (heat treatment) 1999-02-01

6 GB / Z18718-2002 Heat treatment energy-saving technical guidelines 2002-12-01 implemented

7 GB15735-2004 Safety and health requirements for metal heat treatment production process implemented on November 11, 2004, replacing GB 15735-1995

8 GB / T12603-2005 metal heat treatment process classification and code implementation 2006-01-01, replace GB / T 12603-1990

9 GB / T19944-2005 Fuel consumption quota for heat treatment production and its calculation and measurement method 2006-04-01 Implementation

10 GB / T13324-2006 heat treatment equipment term 2007-04-01 implemented, replace GB / T 13324-1991

11 GB / T21736-2008 Technical requirements for energy-saving heat treatment combustion heating equipment

12 GB / T10201-2008 Guidelines for the rational use of heat treatment in 2009-01-01 implemented instead of GB / T 10201-1988

13 GB / T22561-2008 Vacuum heat treatment

14 GB / T22894-2008 Humidity heat treatment of accelerated aging of paper and paperboard at 80 ° C and 65% relative humidity

15 GB / T17358-2009 Power consumption calculation and measurement method for heat treatment production 2009-11-01 Implementation

16 GB / T5953.2-2009 Cold heading steel wire Part 2: Non-heat-treated cold heading steel wire implemented on April 04, 2010, replacing GB / T 5953-1999

17 GB / T5953.1-2009 Cold heading steel wire Part 1: Implementation of heat treated cold heading steel wire 2010-04-01, replacing GB / T 5953-1999

18 GB / T24562-2009 Energy-saving monitoring of fuel heat treatment furnaces

19 GB / T24743-2009 Technical Product Documents Heat Treatment Representation for Steel Parts 2010-09-01 Implementation

20 GB / T15318-2010 Energy-saving monitoring of heat treatment electric furnace 2011-02-01 implemented instead of GB / T

21 GB / T25745-2010 heat treatment of cast aluminum alloy

22 GB / T27946-2011 Limits of harmful substances in the air of heat-treated workplaces

23 GB / T27945.1-2011 Management of heat-treating salt bath hazardous solid wastes Part 1: General management

24 GB / T27945.2-2011 Management of hazardous solid wastes in heat-treated salt baths Part 2: Leachate detection methods

25 GB / T27945.3-2011 Management of hazardous solid wastes in heat-treated salt baths Part 3: Harmless treatment methods

26 GB / T7232-2012 Terminology of metal heat treatment process No. 24 of 2012

27 GB / T8121-2012 Terminology of heat treatment process materials 2012 No. 24 Announcement

28 GB / T9452-2012 Determination method of effective heating zone of heat treatment furnace No. 24 of 2012

29 GB / T28909-2012 Heat-treated steel sheet for ultra-high-strength structures No. 28 of 2012

30 GB15735-2012 Safety and health requirements for metal heat treatment production process Announcement No. 28 of 2012

31 GB / T28838-2012 Code for heat treatment of wooden packaging operations No. 28 of 2012

32 GB / T28992-2012 Heat treated solid wood flooring No. 41 of 2012

33 GB13014-1991 Residual heat treated steel bars for reinforced concrete were implemented on 1992-03-01, replacing GB 1499-1984 ^[1]

Heat treatment

1. Normalizing: heating the steel or steel parts to an appropriate temperature above the critical point AC3 or ACM for a certain period of time and then cooling in the air to obtain

Surface quenching and tempering heat treatment is usually performed by induction heating or flame heating. The main technical parameters are

If the local hardness requirements of the part are high, local quenching and heat treatment can be performed by means of induction heating. Such parts usually need to indicate the location of the local quenching and heat treatment and the local hardness value on the drawing. The hardness test of the parts shall be carried out in the specified area. The hardness tester can use a Rockwell hardness tester to test the HRC hardness value. If the heat-treated hardened layer is shallow, a surface Rockwell hardness tester can be used to test the HRN hardness value. ^[1]

Overheating The overheating of the microstructure after quenching can be observed from the rough parts of the bearing parts. But to determine the extent of its overheating, one must observe the microstructure. If coarse needle martensite appears in the quenched structure of GCr15 steel, it is a quenched superheated structure. The formation may be due to the overall overheating caused by too high quenching heating temperature or too long heating and holding time; it may also be due to the severe band carbide of the original structure, and the formation of local martensite needle-like coarseness in the low-carbon region between the two bands. Caused by local overheating. The retained austenite in the superheated structure increases, and the dimensional stability decreases. Due to the overheating of the quenched structure and the coarseness of the crystal of the steel, the toughness of the part will be reduced, the impact resistance will be reduced, and the life of the bearing will also be reduced. Overheating can even cause quench cracking. If the under- quenching temperature is too low or the cooling is not good, the microstructure will produce a microstructure that is more than the standard, which is called underheating. It will reduce the hardness and sharply reduce the wear resistance, which will affect the bearing life of the roller accessories. Quenching cracks are high or cooling is too rapid. The thermal stress and the structural stress of the metal mass and volume change are greater than the breaking strength of the steel; original defects on the working surface (such as fine cracks or scratches on the surface) or internal defects (such as slag inclusion) , Severe non-metallic inclusions, white spots, residual pores, etc.) stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient or untempered tempering of parts after quenching; cold caused by previous processes Excessive punching stress, forged folding, deep turning tool marks, sharp edges of oil grooves, etc. In short, the cause of quenching cracks may be one or more of the above factors, and the existence of internal stress is the main reason for the formation of quenching cracks. The quenching crack is deep and slender, the fracture is straight, and the fracture surface is not oxidized. It is often a vertical straight crack or ring crack on the bearing ring; the shape on the bearing steel ball is S-shaped, T-shaped or ring-shaped. The microstructure of the quenching crack is characterized by no decarburization on both sides of the crack, which is clearly different from forging cracks and material cracks. Heat treatment deformation During the heat treatment of NACHI bearing parts, there are thermal stress and tissue stress. This internal stress can be superimposed or partially canceled. As the shape and size of parts change, heat treatment deformation is inevitable. Knowing and mastering its changing rules can make the deformation of the bearing parts (such as the ellipse of the ferrule, the size increase, etc.) in a controllable range, which is beneficial to the production. Of course, mechanical collision during the heat treatment process will also cause deformation of the part, but this deformation can be reduced and avoided with improved operations. During the heat treatment of surface decarburized bearing parts, if they are heated in an oxidizing medium, the surface will be oxidized to reduce the mass fraction of carbon on the surface of the part, causing surface decarburization. The depth of the surface decarburized layer exceeds the final machining allowance will cause the part to be scrapped. The depth of the surface decarburized layer can be determined by metallographic method and micro hardness method in metallographic examination. According to the surface layer micro hardness distribution curve measurement method, it can be used as an arbitration criterion. The phenomenon of insufficient hardening of the soft point due to insufficient heating, improper quenching, and improper quenching operation on the surface of the roller bearing parts is called quenching soft point. Like surface decarburization, it can cause severe reduction in surface abrasion resistance and fatigue strength. ^[1]