Cold treatment
Cold treatment can be regarded as the continuation of quenching, that is, the workpiece that has been cooled to room temperature after quenching continues to be cryogenically cooled to sub-zero temperature, so that the retained austenite remaining after quenching will continue to transform to martensite, so as to reduce or eliminate retained austenite The purpose of the body.
Numbers 1 and 2:
The size of the grains depends on the nucleation rate and growth rate.
The first method is to increase the degree of subcooling to control the nucleation rate (speed up) and growth rate (slow down).
The two methods are simultaneous solidification.
With this slow pouring, the overall solidification time is prolonged, and the stirring during pouring can make the temperature of the metal liquid uniform and reach the freezing point at the same time, thereby greatly increasing the nucleation rate and reducing the growth rate. Unlike ordinary castings, the temperature gradually decreases from the outside to the inside to the freezing point, and the dendrites grow to the center of the casting.
At the same time, reducing the pouring temperature can also prevent overheating. Because it solidifies sequentially from bottom to top, the crystallized area is relatively small, and the local cooling rate is greater than that of rapid pouring. And its riser can also be reduced, and the cooling rate is also fast.
Is the martensite obtained by quenching the faster the cooling, the purer (regardless of retained austenite) without ferrite and cementite?
It’s not. Quenching cooling, as long as the cooling rate is less than the minimum critical cooling rate, all martensite can be obtained.
Will slow cooling of quenching oil obtain more ferrite and cementite than quenching water?
Not necessarily. This is the case for steels with less hardenability, but some alloy steel quenching oil will still get all martensite. It mainly depends on whether the cooling curve intersects with the “C” curve. And this “more ferrite and cementite” is generally called troostite or bainite.
Deep quenching should be called “cold treatment”, the purpose is to reduce the content of retained austenite in the steel, stabilize the size, improve the hardness and wear resistance. Generally used for precision measuring tools or cutting tools.
What is the purpose of secondary quenching
The purpose is to refine the grains. Commonly used for carburizing steel. Because steel spends a long time at high temperature during carburizing, and the grains grow, this treatment is often used to refine the grains. The second quenching is not what you said, “I heard that it is quenched in boiling water or hot oil first, and then quenched twice in liquid nitrogen.”
I remember it said in the book that it seems that it should be cooled slowly at the stage of more than 200 degrees to room temperature, otherwise the stress will be large and brittle. So what is the use of this quenching process?
This is because most steels begin to transform into martensite in this temperature range, causing structural stress. And the temperature is low, the plasticity of the steel is very poor, and it is easy to crack. There are many processes to realize this process, such as water quenching and oil cooling, water quenching and air cooling, alternating water and air quenching, graded quenching, austempering, tempering with temperature and so on. Can effectively reduce deformation and cracking.