Can hardened 1144 Stressproof or 4140 ASTM A193 B7 be crackless 1

[kklinger]

Hello group,
I am specifying 2" bar 1144 Stressproof to be induction hardended to Rockwell C 45-55 at case depth of 0.060"-0.080". As an alternate material, I specify 4140 ASTM A193 B7 with the same hardness requirements. Can either of these two materials be supplied without visual and microscopic cracks due to quenching? I understand that the Stressproof material can only be plastic quenched to a max of Rockwell C 55 at max depth of 0.120". Our vendor says that they cannot supply the bar without cracks.

Thanks,

kklinger

 

[TVP]

First, ASTM A 193 is a standard for bolt material. Cold drawn 4140 bar should be specified according to ASTM A 322. If I understand your situation correctly, you obtain cold drawn steel bars from a vendor, and then your company hardens and quenches them. Please correct me if I am mistaken. Based on this, it is necessary to understand more about how the bars are processed and how you control the hardening and quenching processes. 1144/Stressproof is going to be very susceptible to cracking due to the non-metallic inclusions present, but 4140 will be less so. Do you order the bars cold drawn only, or are they ground? Is there any material removal (turning, grinding) performed by your company? Does your steel bar vendor perform any non-destructive testing (eddy current, ultrasonic, etc.) on the bars? Does your company? What are the induction hardening parameters (coil size, frequency, power, scan speed or time)? What is the quenchant type (water, water+polymer, oil), polymer concentration, and temperature? The bars should be free of cracks from the supplier, but may develop cracks due to your process. You need to control all of the above variable very carefully in order to prevent cracking. Can you introduce a grinding process after hardening?

 

[kklinger]

Thanks for your reqly TVP.
We purchase a completed product from a vendor. But as an engineering company we specify the design of the product including the material specs. Our vendor purchases the parts made from these bars from their vendor. And in turn, their vendor sends these bars out to another vendor to be hardened. We had an issue of the 1144 Stressproof material cracking, most likely due to water quenching. The application of this bar can be critical, in that failure of this bar can be catastrophic (loss of life). Therefore, we would like to have the bars to be crack-free, but the bars need to be hardened for wear. As you can see from above, we do not know much about the hardening process. We are not hardening experts; therfore, specifying the heat treatment procedure could get us in trouble since we unknowingly could specify the wrong procedure. Our vendor is telling us that they cannot supply the bars crack-free. Is this a true statement?

TIA

kklinger

 

[NickE]

I just wnat to introduce a question:

With this statement:

"The application of this bar can be critical, in that failure of this bar can be catastrophic (loss of life)."

I am astonished that 1144 steel is speced at all in this case. I would think that the presence of the large sulfides equate to the same as large numbers of cracks, similar to graphite in Grey Iron?

Nick
I love materials science!

 

[unclesyd]

I agree with NickE concerning the use of Stressproof in this application.

Can you comeback with the end application as there might be different avenues of approach to this problem.

 

[oldrunner]

We did have at least two failures of stress proof. One was the base of a mast for a stifflegg derrick that had been "repaired" and the other was a tie-bolt arrangement for a C-hook for lifting curved prestressed concrete girders. The rod broke while the load was in the air, but the crane operator was able to set the girder on the ground before anything dramatic (such as loss of life) happened. When the mast broke, the operator was injured even though he was inside a protective cage.

 

[kklinger]

I think that everyone agrees that 1144 Stressproof was the wrong material to use in this instance (last time I talk to a material supplier rep w/o talking to a metalurgist). Thanks for everyone's responses.

 

[TVP]

1144 is definitely the wrong material. 4140 is a good choice, but as I mentioned previously, it depends greatly on the steel processing, bar processing, and heat treating. It can definitely achieve this type of surface hardness and case depth by induction hardening and quenching in a water + polymer mixture. A typical as-quenched hardness would be 53-58 HRC, which would then be tempered to 45-50 HRC. Optimally the steel would have low sulfur, below 0.010 mass %, but if machinability is needed, this can be increased to 0.015-0.020 %. This type of steel would be produced as a low S grade (below 0.010) initially, and then "sulfur-dosed" in the ladle to the specified level, and finally treated with CaSi wire in order to refine the morphology of the MnS inclusions into a globular shape instead of the long stringer type.

The steel bar needs to be produced to a surface defect limit, and tested by eddy current and/or magnetic particle methods. A typical limit for cold drawn bar that will be subsequently hardened and ground is 0.13 mm maximum. The steel will likely be cold drawn and possibly turned, ground, or ground and polished, depending on the exact needs. All of this will minimize the number & depth of surface defects like cracks, seams, etc. The best cold drawn bars are tested with 2 types of eddy current systems-- one is a comparator method for short defects and one is a rotating probe method for long defects.

Once the high quality steel bar has been suitably manufactured, the induction hardening and quenching processes need to be well controlled. The power should be as low as possible to reach the austenitizing temperature, without overheating. The scan speed should be as low as possible to allow for as much homogenization as possible, and to reduce the possibility for quench cracks. The quenchant needs to be a water + polymer mix, with a controlled concentration of 5-15% (typical) and a temperature > 20 C. Quenching into cold water produces cracks, and if this is the current process, it needs to be changed. The best process would involve grinding/polishing after hardening in order to remove any traces of surface defects.

Based on the above, it is possible to produce millions of induction hardened bars of the requirements you mentioned with exceedingly few cracks. Highly stressed applications should test the fullly processed parts with a non-destructive method such as eddy current, magnetic particle, etc.

 

[NickE]

"and finally treated with CaSi wire in order to refine the morphology of the MnS inclusions into a globular shape instead of the long stringer type."

Is this through the same mechanism as Mg additions to a melt of Grey Cast Iron (flake type graphite will result in Ductile Cast Iron (nodular graphite)?



Nick
I love materials science!

 

[CoryPad]

Yes

Regards,

Cory

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