Induction versus gas fired furnace heat treatment 3

[CdotS]

Does any one have any insight into the steel microstructure (say low carbon low alloy type) that forms after quenching and tempering in a induction heat treatment line as compared to a gas fired furnace system? I am specifically interested in the influence of soaking time (more in gas fired versus a few seconds in induction) on the diffusion of carbon and other elements and how they influence hardness (micro and macro level) and impact properties.

Thank you in advance for your help.

 

[metengr]

CdotS;
ASM Handbook, Volume 4, has an excellent write-up on induction heat treatment of steel, with particulars related to time-temperature for austenitizing, induction hardening and induction through hardening and comparisons with conventional furnace techniques. Excellent reading and much information.

A very brief overview; it looks as though for induction heating, the austenitizing temperature that is used is about 100 deg F higher in comparison to conventional furnaces. The concern over grain growth, which would adversely affect toughness in carbon steels, is not expected to ocurr because of the short time exposure at elevated temperature.

In the section on induction tempering, the hardness response shown for carbon steels yields similar response to conventional furnace tempering heat treatment. The induction heat treatment actually shows slightly higher hardness in comparison to conventional furnace tempering. The conclusion is that the hardness response between the two tempering techniques is very similar and correlates to mechanical properties and fracture toughness response.

There is more information. You really need to review it.

 

[stanislasdz]

another thing,

induction can only treat surface part and it's so hard to treat the heart of the part in the case of thick parts...

 

[Yallapragada1022]

Low carbon steels:
Typically these steels are induction hardened in "As received (normalized or hot rolled)" condition and the hardened pattern microstructure will be martensitic. The core will be ferritic and pearlitic as no hardening takes place below the pattern depth.

Alloy Steels (4150 etc.):

These are normally quenched and tempered before induction hardened. The structure will be martensitic after induction hardening.

As metengr said, grain growth is a concern because of very hight temperature (approx. 2000 deg. F) that is generated for a very short period of time during induction hardening. We always watch for any grain growth and modify the induction hardening recipe to minimize the grain growth. You can detect any overheating while performing metallurgical evaluation of the pattern and correct the recipe by lowering the power and scan speed.

Hardness (Induction Hardening) - Take 4150H as an example:
Macro Level - 60 to 66.0 HRC. (As Qunched) & 58 to 63.0 (after draw). This what we typically achieve on our parts.
Microhardness: You will obtain 58 to 63.0 HRC after draw.

I can add more to this discussion tomorrow. Can you tell us the steel grades that you have in mind. I will be able to add more to this dicussion as I deal with induction hardening on a daily basis.

Hope this helps.

Rao Yallapragada

 

[redpicker]

If you are talking about comparing full section induction quench and temper to gas furnace quench and temper, consider the following.

If the parameters are set-up properly, the microstructure and mechanical properties should be equivalent between the two processes. Some have reported that the induction processed materials will have a slightly higher impact strength than the conventional gas furnace processed materials. In my experience, I have not noticed much difference.

The big issue comes in with the "If the parameters are set-up properly..." part. With the induction processing, the times get compressed considerably, which leads to higher temperatures. As has been mentioned, austenitic grain growth can be a problem. With proper control, however, this doesn't have to be an issue. In fact, because of the shorter heating times, it is possible to have a finer grain size than would be possible with the gas-fired furnace. The key is control.

There are other issues to watch out for, too. Because of the short times, it is possible to not fully austenitize the material (even though the temperature is well above the Ac3). Since most of the Chromium and Molybdenum get tied up in carbides, if you don't dissolve all the carbides, you can get a heat treat response that is considerably less than what would be expected.

Temperature uniformity is another issue that can cause problems. For austenitizing, this usually isn't that much of a problem for two reasons. 1) the heating efficiency drops considerably once the material passes the Curie point and 2) there is a wider accpetable temperature range.

For tempering, however, the uniformity problem can cause problems. Since there is an upper limit for tempering temperatures (the Ac1), to get accpetable hardnesses you have to have an extended soak time. It is with these extended soak times that the temperature variations can lead to unpredictable results.

In my experience, you have to have much more control with the induction processing than with the conventional furnace processing. The induction processing will result in much higher production rates, however, which can justify the cost of the increased control.

rp

 

[CdotS]

Thank you all your valuable inputs. Sorry, I could not respond to your replies earlier.

I am in the process of actually doing some experiments to confirm your theories. Let you know in a few weeks.