Significance of Normalizing Before Q&T for 300M VAR 7

[MechyMarco]

Hi all,

Designing some parts out of 300M VAR per AMS6417 and heat treating per AMS2759/2. I have some general metallurgy questions regarding normalizing and it's effect on the material's response to heat treating.

The raw stock is sent to us in the hot rolled normalized and tempered condition. Why would the mill temper after normalizing? Why not just normalize? I interpret AMS2759/2 to say that normalized stock must be preheated before hardening while AMS6417 makes no mention of that distinction. It just says to normalize and harden.

So my question is how/why does tempering after normalizing affect the outcome. Not a metallurgist but I am really curious about this. Seems like a redundant step as when you Q&T you're overwriting the previous microstructure with austenite anyway so it doesn't matter what it looked like before. Am I missing something here?

Any insight will be greatly appreciated!

Cheers.

 

[EdStainless]

Often with these material the most 'iffy' property is transverse toughness.
If you need to test something that would be on my list.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MechyMarco]

Tbuelna,
Thank you. Yeah we looked at the slightly higher carbon version of 300M but it was a bit of a pain to procure so we settled with the regular version. Any negatives to the higher carbon version? Reduced fracture toughness due to increased carbon?

WKTaylor,
I agree that attempting to through harden 4.50'' sections would be pretty crazy!

EdStainless,
We've currently specified longitudinal tensile testing of three equivalent rounds (diameter = 2x max section thickness) that will accompany the lot of parts during heat treat. Will have a discussion with the boss about potentially amending.

With respect to controlling surface decarb from the mill we will be rough machining off at least .125''. Max thickness of part to be approx .565'' so confident in achieving good through hardening. We've also decided to re-classify the part as "Type 3: Parts with finished machined surfaces or surfaces with less than 0.020 inch (0.51 mm) to be machined off any surface after heat treatment and parts with protective coating on all surfaces." So rough machining will take care of the surface decarb from the mill and the final machining will clean up any critical surfaces for final assembly.

This is my first foray into the world of ultra high strength parts so I really don't want to mess anything up. You've all been incredibly helpful and I really appreciate it.

Currently debating the cost/benefit of doing a final stress relief after final machining. Will be taking off ~.005'' for critical seal surfaces around the gland and rod body. The piston is loaded in pure axial compression at low speeds (treating as static). My reasoning for stress reliving is to eliminate the residual surface tensile stresses induced by final machining. We won't be shot peening (I don't like) so I feel like the potential for a low cycle fatigue crack failure is high. Is the concern warranted? Any thoughts?

 

[WKTaylor]

Stress relief after finish machining... some thoughts.

300M requires two identical ['DOUBLE'] tempering cycles. This ensure 100% grain transformation.

Option 1. IF BOTH Tempering cycles are accomplished on the rough machined part in succession, then a stress relief operation [SRO] on the finish-machined-part is highly recommended.

NOTE. This sequence-scheme is simple and straight forward and hard to mess-up. Also the SRO can be delayed until just before application of plated finishes [can be done by the plater].

2. IF You can split tempering operations there is a nifty way to eliminate the SRO... although testing is advised to ensure no distortion.

Accomplish first temper operation of the rough machined part.

Finish machine the part.

Accomplish second temper operation of the finish machined part.

The second [mandatory] temper operation becomes the defacto SRO. In general the tempering operation HAS to be [should be] done by a certified heat treater. The part then gets routed to the plater who does not have to do a pre-plate SRO.

This routing can get messy and confused between sub-tier processors: ABSOLUTE process clarity and tracking are essential.



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[tbuelna]

After rough machining and prior to HT, it is good practice to round-off all sharp corners/edges. The process can be tedious and time consuming to perform properly, but it's very helpful for preventing quench cracks.

AMS 2759/2 type 3 processing only permits class A or B atmospheres, so decarb should not be a significant problem.

The residual compressive stress from shot peening is helpful for surfaces subject to tensile stress/load cycle conditions that can produce fractures. Based on the description of your piston service conditions above it does not sound like it would benefit from shot peening. In any case, you should never stress relieve a part after shot peening.

As WKTaylor noted above, AMS 2759/2 table 3 requires at least two tempering operations for 300M. His recommendation of finish machining after the first tempering cycle seems like a good idea. There is a requirement in AMS 2759/2 sec 3.4.8 stating "parts should be tempered within 2 hours of quenching" that you need to consider.

 

[WKTaylor]

MechyMarco

1. Tbuelna makes a very important point: 'rough machining' means 'machine to oversize, not to finished dimensions': and definitely DOES NOT MEAN to leave it with ragged/rough-edges and surfaces... which can be crack starters.

Generally speaking: IF You can run your bare fingers quickly all-over the parts and not get abrasions/cuts that's probably ‘good-enough surface-finish for rough machine parts’, at this stage. IF YOU get hurt doing this the surface is too rough to safely SHT.

2. Also, SHT/Q, followed by first temper operation is by-far the critical operation... to stabilize the part in approximately final temper state [W/~80--90% grain transformation completed]. I think-of the second temper operation simplistically as the 'completion-temper'... whatever grain/metallurgy transformation was not accomplished with the first temper operation is completed by the second operation [to reach ~100%]. And it is a stress-relieving operation also.

When the heat-treat, machining and plating processors are widely separated [sometimes hundreds of miles apart], I prefer that the heat treater do BOTH tempers steps to 'giterdone'... then have the part finish-machined[ground]... then have the plater do a true [post machining] pre-plate stress relief operation.

However, one prime vendor we had nearby [who did all machining operations] was just down the street from their heat-treat processor and had a great relationship with them... so doing the rough-machine/first-temper/finish-machine/second-temper process efficiently was common 'stuff' for them [our aerospace work was a 'small job' for them, compared with their on-going local AG/diesel-engine work].

NOTE. Heat-treaters are sometimes called-upon to do interim re-annealing operations when lots of steel-cutting [in the low-temper state] is required... and surface hardening or distortion is noted.

3. CAUTION!! One of the hardest 'elements' that all the processors had was getting their ‘people’ [machinists/techs/transporters/etc] to recognize that on-going flash-rust and impact-damage prevention was serious business! These parts have-to-be-protected by corrosion preventatives and packaging from incidental corrosion and handling damage between operations [oil, CPC, VCIs, barrier papers, heavy duty impact-absorbing wood/cardboard/boxes/crates, etc].

4. NOTE, regarding shot peening...

Generally speaking, for steel and CRES parts HT ~200-KSI [depending] and above, special peening shot and processes are required.

Specify high Intensity peening levels and high hardness steel shot [cast or cut-wire].

Intensity will depend on localized material thicknesses and M&P or specification recommendations.

‘High hardness shot’ [cast or cut wire steel] should be per AMS2431/2 or /8 [and maybe /5], or equivalent. ‘Regular hardness shot’ will NOT actually peen very/ultra-high strength/hardness steel materials.

Also, I always recommend 200% [2.0] coverage, thus… peen it 100% [1.0] all-over-once… then peen it the same way, again!

NOTE.
Peening compresses the surface for fatigue improvements [to retard crack initiation]; and also ‘hardens’ the surface for improved impact and wear resistance.

Some generally useful peening specs & info…

AMS-S-13165 [cx, good info] Shot Peening of Metal Parts
AMS2430 Shot Peening, Automatic
AMS2431 Peening Media
SAE HS-84 Manual on Shot Peening
SAE J2441 Shot Peening
Curtis-Wright [Metal Improvement Co]

https://cwst.com/about-us/technical-articles-product-information/

NOTE: AMS2546 Laser Peening [W/O shot!] might also work… but unsure…





Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[MechyMarco]

WKTalor,
I like the idea of doing the finish machine in between the two tempers. Unfortunately I don't think the machine shop and heat treater can handle the hot potato process. The parts will be moving around the country too much and something would probably go wrong with us ultimately picking up the tab. But a great idea for sure. Something to shoot for on the next batch after we get all parties up to speed on the basics.

tbuelna,
You're right that shot peening wouldn't help me much for most of the piston since it's in compressive loading. But there are a couple areas on the piston that have tensile stresses. Specifically, a port hole and the shoulder between the rod and head. I've done my best to abide by best design practices with large radii and an undercut to minimize the force flow disturbance, but there will always be that bloody port hole. So I was thinking shot peening would help me out in these areas. I'll probably skip the peening since it's not practical to reach into the hole.

Thanks guys. I'll let you know how the finished parts end up performing!

Cheers.

 

[tbuelna]

MechyMarco,

If your port hole and shoulder have axisymmetric profiles, there are other techniques besides shot peening to produce a residual compressive stress in the surface to improve fatigue performance. There are some roller burnishing and fillet rolling tools that can be used with steel at Rc 52-53 hardness like that of your piston. Might be worth looking at if fatigue stress is a concern in those areas.

 

[MechyMarco]

Hey all,

Just got our tensile test specimens back. Results are great:

Average of two test specimens:
Yield - 237ksi
Ultimate - 280ksi
Elongation - 11%
Reduction in Area - 36%
Hardness - 65 HRC

So high five to all of you for helping!