ASTM A193 B7 heat treatment process ? 2

[MCFcorp]

I am currently reviewing our shop traveler for processing B7 studs. The final configuration of the part is an M12 thread X 4.5 in long with dacromet 320 coating.

The parts will be thread rolled in our facility. I am confused as to the proper sequencing of operations to achieve the correct mechanical properties.

Will there be any difference between the two process sequences below in the final configuration?

Seq. 1

Heat treat & quench per ASTM A193 B7
Blast scale
Thread Roll
Temper
Dacromet

Seq. 2

Thread roll
Heat treat & quench per ASTM A193 B7
Blast
Dacromet

I must confess that I am weak in metallurgy. I am unsure if the heat treat process would relieve internal stresses or if the tempering process is necessary in both sequences. There are testing processes in thos sequences.

Thanks in advance for all feedback

 

[metengr]

MCFcorp;
Heat treatment in accordance with ASTM A193 for Grade B7 consists of austenitizing followed by quenching followed by tempering (please review this ASTM Specification). I will leave the remainder of the process steps for you to optimize.

 

[dbooker630]

There are reports of increased fatigue life when you roll the threads after heat treat instead of before. The key is a good rolling practice that does not create laps in the quench-and-tempered material.

I have encountered Q&T to the low side of the hardness range to facilitate rolling after HT, or even a slack quench, so you'll need to ensure that the fasteners have a microstructure that is predominately martensitic (good quench!). Just a precaution.

 

[MCFcorp]

metenger....

is the tempering step a stress relief that is used after cold working? I have read the ASTM spec six ways to sunday and cannot determine a specific or required sequence.

I am basically at a cross roads. Do I thread roll then heat treat with no tempering or do I heat treat, thread roll, then temper? Does heat treating and quenching after thread roll achieve the same properties as heat treating and quenching followed by thread roll then tempering?

The buck stops with me on this issue and I don't have the back ground to know if either is fine or tempering must be done after quenching.

or does thread roll, ht, quench, temper work?

 

[mfgenggear]

MCFcorp

I do not have the ASTM spec Please advise the Core hardness required. Through Harden? & the type of material?
What is the fit form and function of these bolts?

However for manufacturing if the core hardness is 40 HRc or under then I would recommend to thread roll after Heat Treat.then stress relieve 50 deg F below the tempering temp.


if the core hardness is higher than 45 HRc then it will be
advisable to thread roll before heat treat.

Threads will need extra handling as not to be damage during heat treat.

After hardening the Tempering temp. will dictate the hardness of bolts.

I am only given general guidence because I do not have the DATA for my review.

Just my 2 cents.

I am a Senior MFG Engineer not Meturgist or Designer.

Take Care

 

[MCFcorp]

mfgenggear,
My company is producing these studs to fullfill a purchase order to a customer. I do not know the design criteria, just the PO and drawing requirements.

The spec only requires a 35HRC maximum hardness. The material is 4140 alloy steel. The annealed hardness is currently 23 HRC.

ASTM A193 requires that the tempering process be done at 100 degress below the heat treat temperature. For B7 that temperature is 1100F.

The other question I have is...what is the bake cycle time? I am used to dealing with electroplating standards and the specific pre and post bake cycles. Along with the differences for high strength steels (grade 5 and higher). This standard has stumped me but that is probably because of my unfamilarity.

 

[mfgenggear]

MCFcorp

if you are able to obtain a copy of AMS 2759/1 or AMS-H-6875 I would highly recommend it.

it details the temps & time

according to your quote ASTM A193 Heat Treat Temp should be 1100 deg F however the AMS Specification requires the
following for 4140 low alloy steel

Austentiizing should be 1550 deg F (.250 dia for 25-45 minutes min. for atmosphere)

oil or polymer quench (I like using mar quench or Mar tempering) when allowed by the specification.

35 HRc (Approx 150 ksi) within one hour of quench should be tempered to 1025 deg F with thermocouple not less than one hour plus one hour per inch.(will crack if not tempered ASAP)

but you must follow the ASTM temperture to achieve their requirements.

any one else please jump in


Hope this helps

 

[MCFcorp]

it is obvious that immediate tempering is to reduce internal stresses in the material caused by h/t and quenching.

does a second tempering need to occur to reduce stresses induced from cold working the material? in this case thread rolling.

i will review the standards recommended with a little more clarity in the morning

 

[TVP]

MCFcorp,

I think some clarification of heat treating terms would be useful here. Tempering is the process performed after quenching that transforms the as-quenched martensitic microstructure into a tempered martensite microstructure. Stress Relieving is a different heat treatment process that is performed solely to reduce any residual stresses from manufacturing, etc. While some stress relief typically occurs during tempering, that is not the primary purpose: changing the microstructure is.

So, going back to your original message neither Seq. 1 nor Seq. 2 is acceptable. Seq. 1 shows thread rolling after quenching, which is not acceptable because proper threads cannot be formed when the microstructure is brittle, as-quenched martensite. Seq. 2 does not show tempering which is not acceptable. A proper sequence is one of the following:

Sequence A
Austenitize (~ 1550 F for 30-60 minutes)
Quench (heated oil)
Temper (minimum of 1100 F for ~ 60 minutes)
Blast
Thread roll
Dacromet

Sequence B[/b]
Austenitize (~ 1550 F for 30-60 minutes)
Quench (heated oil)
Blast
Temper (minimum of 1100 F for ~ 60 minutes)
Thread roll
Dacromet

Sequence C[/b]
Thread roll
Austenitize (~ 1550 F for 30-60 minutes)
Quench (heated oil)
Temper (minimum of 1100 F for ~ 60 minutes)
Dacromet
*Blast can be after quench or temper*

Keep in mind that blasting is one of the pretreatment options for Dacromet, so you may be able to use only one blast operation to remove scale and prepare the studs for coating. Grade B7 fasteners do not have any requirements for stress relieving, whereas B7M do (see section 6.5).

 

[mfgenggear]

yes the heat treat (must be austentize, quench & temper)
i missed that from the first post. I agree, the parts will
crack if not tempered.its just not done with out temper.

I will throw in some more info
depending on the qty, machined or cast ect

for low qty of parts 10-1000 pcs (screw machine or chucker)

saw the raw bar to 3-4 foot lengths or ?(what wiil fit in oven)
heat treat the bar(hex or round) & straighten.
(max thicknes for 4140 bar is approx. 1.00" dia max will harden through)
blast to clean not required because it will be machined
machine the fasteners (max recomended 4 ft if heat treated)
thread roll
stress (it does not hurt, it can be omited or added )
tumble deburr & clean
coat

for large qty 1500 to 10000?

machine from bar (multi spindle machine or cast?)
heat treat(heat treat can be done before machining if not cast)
blast to clean
thread roll
tumble deburr & clean
coat

Hope that Helps