Effect of Sulfur content in AISI 4150 2

[B1K]

Hello,

Can someone tell me what effect a higher Sulfur content would do to the fatigue properties of AISI 4150. Our current standard is .015 - .040 % and the proposed material is .064%
Thanks!

 

[steelmtllrgst]

Increased sulfur will lower fatigue life by increasing the MnS inclusion density. These inclusions act as crack initiation sites in testing the same way that they do for increasing machinability. However, changing from 0.04 to 0.06% is not going to have such a noticable as would be seen with an already low-sulfur steel (say 0.005, etc.).

 

[TVP]

Please tell me that this proposed grade is to be modified with calcium. If not, the fatigue properties will be quite poor, especially when tested in the transverse directions. Corus has developed some very good free-machining grades with extremely high levels of S, but they are Ca-modified so that the length does not become outrageous. As steelmtllrgst already stated though, the increased density also reduces fatigue life.

 

[B1K]

Yes, the proposed steel is Ca-modified. Even though our standard is .015 - .040%, it usually comes in at the lower end (<.02%) so we will be getting material that has 3 times the sulfur content. We will be producing shafts that have an area that will be induction hardened for a needle bearing on one end, and a drive end with angular contact bearings and a locknut on the other end. My concerns are the induction hardened area wearing earlier than before and any possible breakage problems with the locknut v-threads / undercut areas. Thank you both TVP and stlmtllrgst for the input! I am not a metalurgist and am getting pressure to make the change because the new steel is cheaper. I am not opposed to this as long as the product life does not suffer.

 

[redpicker]

I am not sure that you will not be adversely affecting product life.

Changing from 0.020% S to 0.064% S can seriously affect fatigue life by providing initiating sites at the threads and undercut areas, particularly if the shaft is under a bending load. Depending on the actual geometry and loading conditions, it may not matter, but it could.

The Ca-treatment can reduce the life of the bearing surfaces. It is my understanding that Calcium is detrimental to rolling contact fatigue. I would explore other ways to save money.

rp

 

[TVP]

I would approach this change very cautiously. There is a HUGE difference in the quantity of sulfide inclusions when you go from ~ 0.020% S to anything greater than > 0.040% S. This is not a drop-in replacement. You definitely need to evaluate this from both a product perspective and a processing perspective. Fatigue life is the most concerning aspect, both under bending and under rolling contact. You need to test this, and not just with 1 or 2 parts-- you need something that would be statistically significant. The processing concern is with induction hardening of highly included steels. They are not as forgiving, so be careful with scan speed, quench intensity, etc. If this is high volume production, then process a lot of parts, say 1000 pieces, before making any final determination on processability. Look for cracks on the surface, and cross-section the parts to make sure there are no sub-surface defects.

 

[B1K]

Thanks again everyone for your valuable insight. I will ask that we perform a thorough test before shipping product with the new steel to our customer. However, taking all the comments into consideration, I seriously doubt we will make the switch. The potential cost savings will not outweigh the potential risk.

 

[stanweld]

B1K,
From your last post, it appears that you are purchasing "Out of Spec" material at a discount price. Beware that the high S is the only thing wrong. I would be concerned with laminated plate (if plate is product) or pipe if bar stock. You may well be getting a lot less than you think for your "cheap-at-the-moment" price.