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Up Topic Welding Industry / Metallurgy / Large Forgings - Martensitic Stainless
- - By engineer1984 (*) Date 07-12-2018 23:37
Hi all,

We have thick forgings for shafts (7.5" diameter, finished machining).  We currently have 'too much' stock on the shaft for rough machining that is used during heat treat / quench.  We currently have an extra 1.25" diametrical.  The vendor has suggested 1/4" diametrically.

My questions:

From my understanding, martensite is formed during rapid cooling.  I reason that the center of the shaft may not have any martensite or very little (rapid cooling not taking place).  How deep from the surface will martensite form?  How much martensite is needed for a 400 series stainless (410 in this case) for the stainless to be considered 'martensitic stainless steel'? 

Thanks and cheers,
Parent - - By 803056 (*****) Date 07-13-2018 01:33
Am I correct? You want to quench the shaft before machining? What are you going to do, grind the shaft to the right diameter?

What hardness are you shooting for and what cutting tool are you planning to use to cut this shaft after hardening?

Parent - By engineer1984 (*) Date 07-13-2018 17:44
We forge, pre-machine, heat treat, then final machine.

We are shooting for hardnesses, tensile, etc that meet ASTM A479 410 Condition II.  We plan on the pre-machining leaving an 1/8" to a 1/4" radially of stock.
Parent - By engineer1984 (*) Date 07-13-2018 17:58
To be fair, I may not understand the process correctly.  Maybe the martensite forms throughout the entire shaft, but I understand martensite formation to be dependent on the rate of cooling, not just temperatures.
Parent - By engineer1984 (*) Date 07-13-2018 18:10
1) I'm not a machinist, but I'm assuming tungsten carbide?   It's going on a lathe to get cut to finish 64 or 125 for most of the shaft.  There are bearing journal areas that will be ground to 32 finish.  I don't know the machining tool material though, I could be wrong.  How does this impact the material properties?

2) I'm starting to come to the conclusion that the amount of martensite dictates the tensile strength and hardness.  Therefore, one can use a tensile strength and hardness measurements to understand if the martensite to austenite ratio is correct.  But on a thick shaft like this, I'm guessing that there is no way to make the center of the shaft truly 410 stainless steel.  That's fine, the center of the shaft doesn't see stress.  The question then becomes: on a scale of 1 to 0 measured from the Outer Radius to a Radius = 0, how do the shaft materials change?  I think this is largely a empirical question, but didn't know if there were equations, rules of thumb, etc that might describe how the properties might change within the shaft.
- - By 803056 (*****) Date 07-13-2018 21:22
The stainless steel is a martensitic stainless steel regardless of the state of heat treatment. The term martensitic stainless steel is used to describe the family of stainless steel. Any of the martensitic stainless steels will respond to a quench because they contain sufficient carbon. This is in contrast with the ferritic stainless steels that may contain similar percentages of chrome, but insufficient carbon to respond to quenching.

The maximum hardness is a function of the maximum austenitizing temperature and the quench rate. The quench rate is dependent on the quenchant; air, oil, water, or brine. There are synthetic quenchants available, but they are often compared to the "traditional" quenchants I listed.

Martensitic stainless steels can be air quenched and attain a high hardness. As you noted, there is a relationship between hardness and strength. The maximum strength is reached when the maximum hardness is reached. However, the maximum quench rate can also result in quench cracks, so there must be a balance that is maintained.

As you noted, the highest hardness readings indicative of a martensitic microstructure  will be on the outside surfaces of the shaft. Less martensite will be formed toward the center of the shaft. One of the advantages of the martensitic stainless steels is the depth of the hardening. But still, with a large diameter shaft, it is unlikely the full thickness will be fully hardened.

Rough machining would be usually accomplished while the type 410 martensitic stainless steel is in the annealed condition. Once rough machined, it could be heat treated to harden it and then the shaft ground to the final dimensions. Usually, the areas that would receive finish grinding would be those areas that receive bearings. Then again, I obviously don’t know what the shaft will be used for, the reason the shaft will be hardened, or the tolerances involved.

Parent - - By engineer1984 (*) Date 07-17-2018 20:42
Thanks for your reply.  I'm enjoying the lessons.

So am I correct in stating that 'martensitic stainless steels' are different than 'martensite'?  Martensite is the actual crystal structure (BCT) after austenite has been rapidly cooled.  Martensitic stainless steels are 'just' a chemical composition that would allow martensite to form if the quench was done correctly?

We had a shaft that was 410 stainless steel composition, but it formed intergranular corrosion during operation (acetylene process with acrylic acid (pH = 4.5) and excess carbon).  The lab tests show there was more austenite in the corrosion area than is normal in a 410 shaft.  My conclusion, perhaps incorrect, is an excessive amount of pre-machining stock during quench.  During final machining, the martensite was machined away allowing the intergranular corrosion to take place. 

This theory would make sense if the quench mainly effected the outer 1/2 inch or so of the material being converted to martensite (due to cooling rates). 

Do you think the theory of excess stock during quench would cause the final machined part to have too little martensite?

Or perhaps it is as simple as an improper quench when the shaft was made.
Parent - By 803056 (*****) Date 07-17-2018 21:27
Martensitic stainless has the crystalline structure typical of martensite, i.e., body centered tetragonal once it is heat treated.
In comparison to carbon steel the depth of hardening is enhanced by the addition of alloying elements, but the depth of hardening isn’t unlimited. The volume of the quench tank must be large enough to ensure the required quench rate is achieved. So, assuming the depth of hardening is limited, excessive diameter that is subsequently removed once the quench is preformed will reduce the depth of hardened material remaining. That is one reason I would expect the shaft would be rough turned to nearly the required diameter before the heat treatment is performed. The other reason to machine before hardening is to make the machining operation easier and less costly.

Parent - By Eutectic (**) Date 08-07-2018 11:47
The answer to your question lies in understanding the Jominy end quench test for a specific composition.
Up Topic Welding Industry / Metallurgy / Large Forgings - Martensitic Stainless

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