Carbide size control in steels 2

[CdotS]

Can any one help me? I would like to control the size of iron carbide particles in 0.8% carbon steel. Can I adjust the austenitizing temperature and soaking time to control carbide size? Will the same technique be useful for low alloy steels where other carbides (e.g. chromium) also present.

 

[metengr]

Cdots;
The austenitizing temperature and soak time will affect the austenite grain size of the steel - the higher the austenitizing temp or the longer soak time will result in a larger austenite grain size.

Carbides are precipitated during cooling from the austenite region for carbon steel and low alloy steels. If you desire to have a fine pearlite structure (closely spaced alternating rods of iron carbides in a ferrite matrix), a controlled rate of cooling can achieve this. You should refer to a CCT diagram for your particular steel.

If you desire a uniform distribution of iron carbide particles, you would need to perform a quench and temper heat treatment. The steel would be quenched from the austenitizing region and reheated to a lower temperature to form iron carbide particles. Depending on the tempering time and temperature, you can control the size of the iron carbides to achieve the desired mechanical properties.

 

[CdotS]

Metengr

Thank you for your useful post. I understand that we can control the size of Fe-C particle in the martentic microstructure through tempering temperature and time. the question now is "How do we control the size of other carbides such as Cr-C particles?"

My feeling is that these alloy carbides do not necessarily dissolve in austenite when soaked for a short period during induction hardening. I am interested in controlling these carbide size in a induction heating, quenching and tempering process.

 

[metengr]

CdotS;
Chromium as an alloying element is soluble in iron and also constricts the size of the austenite phase field, this means it is a ferrite former. Carbon is also very soluble in austenite, and is an austenite stabilizer - e.g., it reduces the lower critical transformation temperature.

You will have to probably increase the austenitizing temperature well above the lower critical transformation temperature to assure that you have complete austenite formation. If your chromium content is below 5 w% with 0.8 w% C, austenitizing at 1750 deg F min should suffice. If you are using induction heating, you should allow sufficient time to get the surface of the part at 1750 deg F min for the necessary time to form austenite. This may take some work with coupons to optimize the induction heating time at temperature to achieve the proper temperature gradient.

As mentioned earlier, to produce a uniform distribution of chromium carbides will require you to quench from the austenitizing temperature and temper to form the chromium carbides.

 

[metengr]

Cdots;
As a follow-up, I found this article on Industrial Heating magazine web site. It applies directly to your question regarding carbide dissolution, and should be of help;

http://www.industrialheating.com/CD...tures/BNP__Features__Item/0,2832,9966,00.html

 

[TVP]

First, excellent job metengr! It is rare that one can so readily access a web page that provides data in such a direct manner.

Second, as the curves demonstrate, it is impossible to obtain a homogeneous austenite without residual carbides when using induction hardening-- the kinetics of diffusion just don't allow sufficient mobility for Mo & Cr. Homogenizing treatments are always long time (hours) soaks at temperatures well above the lower critical.

 

[CdotS]

metengr, excellent source of info which is practical as well. Many thanks.