case carburising 3

[neets]

hello all,
i have written this query before but due to technical problem with login i couldn't view the opinions.
so here it is again

This is specific to case hardening of 16MnCr5, what could be the probable reasons for getting retained austenite and carbide networks in the case. the required depth is 0.3 to 0.7 and hardness is 58-63HRC.

what is the effect of Retained Austenite?

what is the effect of presence of carbides?

 

[GRoberts]

I imagine what you have there is an austenitic manganese steel (Hadfield) even though the ones I am familiar with have a little lower Mn and Cr. Since Carbon is also a austenite former, I don't see why you wouldn't have an austenitic microstructure in the case. What temperature are you case hardening at? If it is within the sensitization range of stainless steel, my guess is that the carbides are chromium carbides that form at the elevated temperature. What is most interesting though, is why are you case hardening an austenitic manganese steel. These steels are used because they rapidly work harden, so case hardening is not normally necessary. What would the surface hardness be if you shot peened the surface instead?

 

[TVP]

16MnCr5 is not an austenitic manganese steel. It is the designation used in the European standard specification EN 10084 for a case hardening steel with the following composition limits:

C: 0.14 to 0.19
Si: 0.40 max
Mn: 1.00 to 1.30
Cr: 0.80 to 1.10

Retained austenite is always present in significant amounts in the cases of carburized steels that are quenched to room temperature. This is due to the increased carbon content which significantly lowers the Ms (martensite start) temperatures and depresses the entire temperature range for martensitic transformation to below room temperature.

The carbides that form upon reheating (tempering) are another common feature of carburizing. As the steel is heated, the carbon content of the austenite is reduced, Ms is increased, and the resulting structure has a reduced amount of retained austenite.

You need to determine if the amount of retained austenite is excessive and if the carbides are "massive". If the amount of retained austenite is greater than 50%, then the resulting hardness will be significantly lower, as will the fatigue strength. The most common cause for this is too high a surface carbon concentration. This is also the cause of massive carbides-- these carbides form during the high temperature stages of carburizing or when the temperature of the part is lowered to ~ 845 C just before quenching.

You should consult some references on the heat treating of steel for some background on the case hardening process (carburizing). I highly recommend ASM HANDBOOK Volume 4 Heat Treating. There are several articles on the various processes (gas carburizing, pack carburizing, etc.) and the resulting microstructures and properties. You can obtain this book from ASM International at

http://www.asminternational.org

. They will also have other references on heat treating, that may be of use to you.

 

[metman]

To: neets- As TVP says, "determine if the amount of retained austenite is excessive." In some instances, 15-20% retained austenite is benificial or optimum (preferred). Yes the high Carbon concentration forms at the high temperature of carburizing but Carbide agglomeration and networking occur near the critical temperatures and especially if held between the upper and lower critical temperatures (also see my response to neets query above which I responded to before reading this thread).

Leonard Jesus is the WAY

 

[neets]

this is in addition to the same question with some more doubts!!!

the heat treatment given was 920deg C 2hrs 30 mins, 840 deg 45 minutes, oil quench!!

i have retained austenite > 50% till 0.3mm depth and carbide network till 0.1mm,

what could be done to reduce the retained austenite and carbide networks... any suggestions in the process!!

 

[TVP]

The carbon concentration at the surface of your parts is TOO HIGH. I do not know what type of carburizing treatment you are using (gas, pack, liquid, vacuum, or plasma), but I would start investigating the atmosphere controls currently being used. How is the carbon concentration being monitored and adjusted? Is the correct set point being used? Are you using an outside vendor for the heat treating, or is this done by your company? It sounds like you need a heat treating expert to review this process. Best of luck.

 

[neets]

In addition to the same, Its gas carburising and
i would like to know today what are the methods and instruments available to measure carbon concentration and the set points.

please tell me the expensive as well as non expensive ones? suitable for which kind of furnace(s)?

what is generally being used in the heat treatment industries very commonly? and to what extent can we fetch information?

 

[TVP]

neets,

Heat treating equipment is outside of my area of expertise. I recommend you consult with someone more familiar with this topic. The first place to look for more information is at the website of the ASM Heat Treating Society (

http://www.asminternational.org/hts/Index.htm).

The monthly magazine they publish is called Heat Treating Progress, and it is the best source of information on suppliers in the heat treating industry.

As an example, there is an article in the April 2002 issue of HTP that is directly related to this topic:


"Atmosphere Monitoring & Control" authored by
Robert T. Oesterreich, Shahab Kazi, Richard F. Speaker, and John E. Buonassisi of Air Liquide America. A summary of the article reads, "This basic guide to instruments for atmosphere analysis explains how each is used to help optimize heat treating processes. The various levels of control also are discussed."

The magazine also has a Internet Directory with which you can search for companies involved in heat treating. Follow the link below, and then select "Heat Treating Equipment and Supplies" on the Companies involved in tab, and select "Commercial sites" on the website type tab.

http://www.asminternational.org/Template.cfm?Section=InternetDirectory&NavMenuID=662

 

[evmundo]

All of the above are true to some extent. The process that you need to employ for this is a boost and diffuse cycle, wher you would load the surface with a high carbon content atmosphere and then diffuse this in towards the core at a much reduced carbon potential atmosphere. this will control the case properties and eliminate most of the retained austenite and carbides. retained austenite is not generally desirable as in some instances it will transform under work at the surface to produce untempered martensite, which could cause detrimental effects during use. The effects that you are seeing are not the norm for heat treating these days with the advance in computer control of furnace atmospheres, carbides would not normally be seen, as mentioned earlier these would act as stress raisers with in the component, leading to cracking and premature failure in use. I would recommend employing a sealed quench furnace that controls the furnace via an Oxygen probe. try Bodycote Heat Treatment for more details 01922 453388.