Heat treating of AISI 1070 versus AISI 1074? 5

[coreman73]

Hi all,

I currently have two ratchet cable cutter blades. The first was formed by hot forging of AISI 1070. The second was formed directly from AISI 1074 plate by wire EDM. During service, the forged parts are routinely cracking while the EDM parts are not.

I analyzed the bulk chemical composition of each sample and both matched to their respective material grades. However, the AISI 1070 blade additionally contained a Cr content of 0.134 wt% along with Cu of 0.167 wt%, which are not part of the 1070 specification.

My question is if the AISI 1070 sample was heat treated in standard fashion for this type of steel, would the Cr and Cu contents prevent proper heat treating? In other words, would the heat treat schedule have to be adjusted for the inclusion of these two alloying elements?

I would appreciate any help greatly.

Thanks!

 

[metengr]

coreman73;
The one piece of advice I have for you to consider is to determine the root cause of cracking in the forged, 1070 blades. Get a sample pf the cracked blade(s) and have a proper metallurgical examination performed before going down the possibly wrong path of questioning heat treatment and chemical composition.

 

[coreman73]

Metengr,

I am actually going to begin metallurgical examination of the cracked blade today and will be comparing it to the EDM. I will post my results as I get them. But just in theory, could the extra two elements discovered in the 1070 affect heat treating in any way?

I agree though and don't want to take the wrong path either. The chemical results were obtained first so I figured I'd try to find out if it was as least a possibility for consideration.

 

[redpicker]

The AISI specifications allow for residual levels of Chromium of 0.20% max and Copper of 0.35% max. You are below these levels.

Depending on the heat treating process, these residual levels could have a minor effect the heat treating results, but without knowing the specifics of the heat treating, it is pointless to try to predict if this effect would be beneficial or detrimental to cracking tendency. As metengr states, you need to first identify the root cause of the cracking.

rp

 

[TVP]

coreman,

Those levels of Cr & Cu are well within typical ranges for a carbon steel grade like 1070, and it would be exceedingly rare that they would have a detrimental effect on the properties/performance of a properly forged part. As the others recommended, a thorough metallurgical analysis will identify the root cause.

 

[coreman73]

Redpicker,

The heat treat process consists of two separate heat treats for each blade type (forged and EDM). The 1st heat treat is to harden, quench and temper the overall blade to 42-46 HRC. The second heat treat is to induction harden the cutting edge profile only and temper to achieve an HRC of 61 minimum.

The only difference between blade types is that the forged sample was normalized before machining and heat treating.

The vendor is asking to determine why the EDM sample worked and the forged sample cracked (on the cutting edge).

Another note is that the forged sample has a thickner cutting edge than the EDM.

Thanks for the input so far guys. I'm glad I've found a place for quality help with these type issues.

 

[coreman73]

I've done the metallurgical analysis on these two blades with results as follows. The specifics of heat treat are listed in the above post.

Forged blade:
1. chemical composition matches to spec
2. microhardness of cutting edge tip area shows all values above the required minimum of 61 HRC; no surface case
3. crack path is intergranular

EDM blade:
1. chemical composition matches to spec
2. microhardness of cutting edge tip area shows a decarburized layer of approximate depth 0.010"; hardness of 48 HRC at surface with values gradually increasing until leveling out at ~60 HRC at 0.010"
3. decarburized layer found around full perimeter of part including area not affected by induction hardening.

To reiterate, the forged blade failed and the EDM blade did not. It's interesting because the forged blade demonstrated the correct cutting edge hardness while the EDM blade had the lower hardness/decarburized layer.

Both blades were used to cut 954 kcmil ACSR cable with hardness of 51 HRC.

Please let me know what you guys think. Any and all opinions are welcome.

 

[dbooker630]

What is the grain size in the induction hardened area adjacent to the crack path?

 

[metengr]

What was the quenching medium for the forged blade and the EDM blade? The reason I ask is that the intergranular crack morphology that was reported is not what I would have expected.

What was the appearance of the fracture surface before metallographic examination?

 

[coreman73]

dbooker360,
I was not able to delineate enough grains to give an accurate grain size. In fact, I barely saw any grains within the induction hardened zone where the crack was. I was able to see grains but they were primarly in the not induction hardened areas.

metengr,
I probably should have been more precise about the crack mode. For some reason, I didn't have much success getting the grains but from the few I saw around the crack was that it appeared to be intergranular. So I am definitely not 100% sure on this.

As for the fracture surface appearance before metallographic examination, I was not able to see it. This part only demonstrated one VERY tiny microcrack on the cutting edge surface so it was not feasible to section it in order to see inside. I simply made one longitudinal cut through the crack and mounted that. I realize it's missed information but unfortunately it just wasn’t possible.

The etchant I used for revealing the prior austenite grain boundaries is aqueous picric acid with wetting agent. I will make more attempts.

I will ask the vendor about the quench medium used.

 

[dbooker630]

At least you can still use picric - my company won't allow me to use it or store it!

 

[coreman73]

dbooker630,
Well, my company doesn't allow the picric powder either. I have to buy the saturated aqueous picric acid solution instead.

metengr,
The same quenching media was used for both forged and EDM blades:
Oil following austenitization during normal heat treat.
Integral polymer following induction heating.

Also, there was no evidence of oxide along the inside of crack path. The core microstructure was fairly homogeneous inside the induction heat affected zone for both blades and was tempered martensite.

Review of the longitudinal sections indicated a banded core for the forged blade and heavily dendritic structural orientation for the EDM blade.

I'm stumped on the root cause of failure. Could it simply be that the forged blade needs to have a thin layer of lower hardness (to increase toughness) prior to reaching the higher required hardness range of 61 HRC? This is the case for the EDM blade, which did just fine considering it has a thinner cutting edge (0.020") versus the forged blade (0.046").

 

[swall]

From what I see here, you are getting quench cracking. Quench cracking with induction hardening is sensitive to configuration, among other things. Are the induction hardening parameters the same for the forged parts and EDM parts? You may need to tweak the parameters for the forged pieces.

 

[Wrenchbender]

Without the benefit metallo/fractography, and just going by the observations so far, my $0.02 is a combination of four factors listed below. Forged vs plate may not be especially relevant; both part are wrought. (Also you're aware of the directional properties of plate.) So eliminating this variable, its just parts 'A' and parts 'B'.

All else equal, detrimental factors in 'A' cutters include:
1. Cu is a trash element whose presence does nothing for fx toughness, and .17% may be too high.
2. Chromium carbides, while great for hardness, can nucleate cracks. More Cr increased that probability.
3. Normalizing may not have been necessary and may have caused grain growth = lower toughness.

Positive factors in 'B' cutters:
4. EDM process has obviously caused a decarb layer to form in the wire's path. This made the cutting edge tougher than part 'A' (48 vs 61 HRC). This might be a good thing in disguise. (Soft edge flattens out again the harder cable until the interior picks up the load …?)

Don't know enough to comment on the apparent intergranular nature. Also you might consider NDT for cracks somewhere in the mfg process until a cause is ascertained.

 

[swall]

Bestwrench--I would have to go with TVP here, that the Cr and Cu levels are largely irrelevant.

 

[redpicker]

I agree with swall, both on it is likely quench cracking (based on the information provided) and that the residual levels of copper and chromium are not likely to have a direct effect on the cracking. Most certianly, the higher carbon content of the 1074 would have a greater affect than these residual levels. Normalizing prior to Q & T is done to refine the GS, so it is unlikely to resulted in grain growth.

Are we dealing with two different heat treat batches, one that has seen a lot of cracking and one that has not, or do the cracked forged parts come from multiple heat treat batches and the EDM parts come from multiple batches? It could be just a matter of when the forgings were processed, they sat around in the as-quenched condition too long while the EDM parts did not. This isn't likely if the cracks appear from multiple batches, but if all the cracked parts came from the same heat treat batch (and the EDM parts came from a different batch), that could be important.

rp

 

[coreman73]

Thanks so much for all the exerienced advice guys. I have some new information from the vendor.

First of all, he said that the heat treat parameters were exactly the same for both the forged (1070) and EDM (1074) blades.

He had mentioned that there was also a CNC'd version of the blade made from 1074 that performed well. He confirmed that the 1074 blades outperformed the 1070 blades.

As far as manufacturing/batch times, he said that the EDM blades were processed several months prior to the forged blades.

From this information, could the material grade used be a contributor to failure? Maybe the residual copper and chromium levels really have contributed to the poor performance of forged blade? Or maybe these two steels really do require different HT parameters. Just to reiterate, the only difference in chemical composition is that the 1070 blade contains % of these two elements while the 1074 blade is virtually completely lacking. The carbon level is nearly identical between the two steels.

I do agree with the quench cracking though since the crack site does not show any signs of damage/deformation or surface notches/abnormalities.

 

[swall]

coreman73--you have to let go of that material thing! The steels conform to AISI/SAE spec limits and you have no control over residuals as long as they also conform unless you want to write your own specs and buy expensive material. I would ask the heat treater what his recommendations would be on fine tuning the induction hardening process to mitigate quench cracking. I am thinking perhaps that increasing the quench delay and/or cutting back on the power setting for the forge pieces might be the place to start. I would also make sure that he has his quench polymer monitoring under control.

 

[redpicker]

Agreed. Although, you can get low residuals without resorting to expensive material. Depending on volume, you can pick and choose from what a warehouse has in inventory, for example. It could increase cost and lead time for the raw material, but it might be that much of an increase, just more trouble. However, it is very unlikely that these residuals are related to the problem, so you would be increasing cost and effort for no benefit. The residual limits permitted by the AISI standards exist so that the levels of residual elements do not affect the performance of the material. The limits would not be 0.35 max Cu and 0.020 max Cr if these levels would lead to problems.

There is something that isn't right. If the heat treatment parameters are exactly the same, why does the forged blade exhibit no decarb while the EDM blade has approx 0.010" decarb?

rp

rp

 

[emonje]

"why does the forged blade exhibit no decarb while the EDM blade has approx 0.010" decarb?"

May be because EDM causes much higher localized temperature rise than forging?