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The impact to the organization and property of the heat treatment of D2 steel's quenching and backdraft

preface

Dongguan Xinzhan Ultra-cold Technology Co., Ltd.has been aksed some questions to explore,the main opinion of the question is that the customers don't trust the discription "when process the ultra-cold to D2 steel after quenching and tempering treatment ,will increase the hardness and deformation due to excessive residual austenite and release residual stress."
Therefore,Xinzhan gave 9 samples to us for analysing ,in order to solve the problem as just mentioned.
1.Basic theory
In order to clarify the question that as previous mentioned, it is necessary to clear some of the basic theory that recognized as an undisputed.
A The impact and phase change of Heat Treatment
1.For example to D2 steel mould, the procedure of quncing cooling after the temperature increase to the certain degreen of austenite,will appear the deformation from austenite to martensite.
Ultral-cold treatment is the continuous of the incomplete deformation (Changde from residual austenite to quenching martensite )
2.The process of deformation will together with volume effect, that is austenite special volume(cm³/g)is small(0.1212+0.33%c),on contrary ,the Martensitic's volume is larger (0.1271+0.025%c),that lead to volume expantion when austenite trun into martensite.
All from the austenite into martensite quenching,the hardness will increase.Because the ultral-cold treatment make residual austenite change into martensite as much as possible,and the hardness will increase after ultra-cold treatment. However,the increased rate is different from the volome of residual austenite.It must have the process of backfire after ultra-cold.and then from quenching martensite to backfire martensite,the previous hardness may decrease. The higher of the temperature of backfire , the hardness will decrease more obviously.
The technics and phase change of quenching and craft.
During the quenching and backfire to improve the temperature (and the soaking time,the major is heating) have important influence to the property after the phase change. D2 steel is an example:
1. Quenching temperature (austenite temperature) on the hardness and residual austenite as curve in Figure 1. Shows that D2 steel use to use 1020 ~ 1050 .c can obtained the 63 ~ 64HRC of hardness and 14 ~ 23% of the retained austenite, the higher is the quenching temperature , the more of retained austenite.The above mentioned basic theoretical and experimental results,the two sides of argument must have common language and standards.
2.Samples,Heat treatment,Ultra-cold Craft,Hardness, Microstructure and Eutectic carbide
The 9 samples (150*100*20MM ) that Xinzhan Ultra-cold Technology Co., Ltd.has offered,the heat treatment and ultra-cold treatment craft as follows:

3.Result analysis
1. The unevenness of eutectic carbide of steel meet the technical standards
2. From the sheet1,the hardness of 3 categery and 9 samples are 58.5+-0.5HRC ,the hardness does't increase after ultra-cold treatment.,why?
We have mentioned in the basic theory:Ultra-cold treatment is to make the low hardness of residual austenite to turn into the higher hardness of martensite after quenching and backfire.Obviously,the premise is how many residual austenite before ultra-cold treatment? The amount of retained austenite related to quenching temperature and tempering temperature (and frequency) .There is two element to influence the volume of austenite:
I.Adoptint the low quenching temperature (1020.c),that is the austenite alloying is low,the volume of austenite is less (about 12~13% )
2.Processing the backfire (500.C),most of the austenite had turned into martensite (500.C,one time backfire,the residual austenite is about 3% after ultra-cold treatment,tempering after 2 to 3 times,the amount of residual austenite become less than before),Therefore, even if the residual austenite about 3% all turn into quenching martensite after ultra-cold treatment, there is minimal impact to hardness
If use 1050 .c for quenching.the amount of retained austenite up to 23%, and the tempered by the lower temperature (eg 400 .c or less) ,the amount of residual austenite is large, the hardness of mould is not high (about 57 ~ 58HRC),then if process the ultra-cold treatment ,more residual austenite will turn into martensite,and the hardness will increase.
4��About deformation
Mechanical parts, molds and other frequently during the placement and use of difference sizes, deformation or even cracking, which relate to the stable organization, backfire not enough or the stree does't eliminate after heat treatment.For example,unstable residual austenite will turn into quenching marstesite in a certain external conditions (temperature,stress),due to additional stress in tissue volumn,and the quenching marstesite is an unstable organization,withour the appropriate temper,the brittleness is high,and also have damaging.
So on the precision parts,mold deformation,overproof,more retained austenite after heat treatment,one of the reasons is the unstable organization.After ultra-cold treatment,the residual austenite will turn into quenching martensite as far as possible,and it is an effective method of stabilizing the organization after quenching martenisite and tempering again.of course,precision mold and parts duing the placement and using will appear overproof,deformation, and one of the element that make the tissue unstable is over amount of residual austenite.The metallurgical quality of steel,for example,the homogeneity of high-chromium will influence the distribution of residual austenite after heat treatment and ultra-cold treatment,also is another element influencing the deformation degreen.
Conclusion:
The submit for censorship of the steel was not found the phenomenon that the hardness improve after ultra-cold treatment,it is because of the original lower quenching temperature, a relatively small amount of residual austenite after quenching ,in addition,it had backfired before ultra-cold treatment,small amount of residual austenite,so the cold treatment is also a small amount into martenisite,the hardness is minimal.
The premise that make the hardness increase after ultra-cold treatment is:there is large amount of residual austenite before handling ultra-cold treatment,while the amount of residual austenite is relate to the heat treatment and backfire.
The residual austenite is an unstable organization,its existence will influence the stability of the organization,during the process that turn into martensite ,this phenomenon will occur,due to the changes in specific volume.We should minimize the volume of austenite to the precision parts which have size requirements.
Mechanical Engineering Department Of South China University of Technology

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