Post heating and PWHT (Post weld heat treatment)?

Date 09-03-2010 14:55

Post Heating serves the same function as preheating. It holds the temperature of the work at a sufficiently high level (usually the minimum interpass temperature) so that the weld will cool slowly and evenly.

Post Weld Heat Treatment can be done after the weld has cooled, this can be Stress Relieving, Annealing, or Normalizing, and is usually performed to a procedure.

Post Heating, if required, should be noted on the WPS. Post Weld Heat Treatment, if required, should be noted in the design drawing.

By 803056

Date 09-03-2010 15:26

I disagree to some extent with what has been stated.

Post weld heat treatment is a controlled process that has to be done properly if the desired results are to be obtained. Different metals respond differently to different heat treatments. PWHT is an essential variable that has to be addressed when the welding procedure is qualified and it has to be addressed by the welding procedure specification. This is true regardless of the welding standard that applies to the work.

PWHT can include tempering, stress relief, normalizing, or annealing. Some base metals have to be PWHT as soon as the welding is completed and other base metals have to cool to room temperature before PWHT can be initiated. An example of the latter would be when PWHT is required for certain high strength low alloy steels to ensure the austenite has decayed to form martensite before the PWHT operation is performed. A shop skill it is not.

When PWHT is required, a separate PWHT procedure is warranted. It should not be left up to the shop personnel to determine how hot the oven is to be when the weldment is placed into the oven, the rate of heating, the length of time the weldment is to be held at temperature, the cooling rate, and at what temperature the weldment can be removed from the oven, etc. As already mentioned, different metals respond to heat treatment differently and that knowledge is beyond the scope of training and experience most shop personnel.

Best regards - Al

By L51174

Date 09-03-2010 15:57

I agree, my answer was meant to be very general, addressing the question, what is the difference between PH and PWHT.

By RonG

Date 09-03-2010 16:16

PWHT requires a chart showing Start/Rise rate /Soak/Fall rate/End. A Post heat we like to call a Hydrogen bake out just means to maintain Preheat and slow cool.

When our WPS's require PWHT all the above info is listed on the WPS. If's it's Post heat it' requires us to establish and maintain preheat, wrap in an insulating blanket and slow cool.

By tigerlee Date 09-04-2010 03:57

Thank you for reply everybody.

Dear Sir,

You mean, any heating method and temperature should be written on WPS before or after welding.
But At shop, during fabrication (marking, cutting, butt welding) they are difficult to control for the straightened and prevent distortion.
If occuring distortion It is difficult to align to butt joint. So, some specialist(experienced fabricator) involved and they are straightening by heating normally.
In this case, How to written heating control method on WPS.

and AWS D1.1 C8.5.5 Heat Straightening. Heat straightening of steel
members requires the sequencing of various heating patterns.
Literature (References 1, 8, 9, 13, 14, 15, 16, 17,
and 18) exists that provides guidance as to the mechanics
of heat straightening. The actual process remains one of
operator experience.

according to Above sentance of AWS D1.1, what is mean that " The actual process remains one of operator experience"?

PLEASE ADVICE ME!!

By RonG

Date 09-04-2010 15:22

Now I am unsure of your question!

A PWHT is an Essential Variable in a WPS for the purpose of STRESS RELIEF (residual welding stress's) primarily. What you discribe is not necessarly the same thing and yes it requires some experience.

So does the PWHT for that matter.

You need to understand the difference between STRESS RELIEF and HEAT TREAT.

Aligning joints does not necessarly require heat, if you are indeed working with High Strength material and you starting heating and bending then you will most likely need a HEAT TREAT to restore the required condition of the material.

In its simplest terms a Stress relief will never exceed the Tempering temperature of the base material.

What the heck are you building?... What material? ......and Why?.... Are you a Welder/ Engineer/ Inspector???"? Sure hope your not building a bridge or building (in the US anyway).

By 3.2 Inspector (***)

Date 09-05-2010 03:12

So you often STRESS RELIEF at temperatures below 500F ?

3.2

By RonG

Date 09-05-2010 19:55

3.2 (Slim), was that question for me? I am not sure what the 500'F represents. If you mean the tempering temp of 500'F (pretty hard stuff):

Then yes! For the purpose of relieveing residual stress you can not exceed the tempering temp. AND yes a PWHT (stress relief) can be accomplished at that temp.

Thermal stress relief being a "Time and Temp Phenomonem" (bet I mis-spelled that along with some others). At 500'F you would need to greatly increase the time at temp to achive Stress Relief.

By 3.2 Inspector (***)

Date 09-06-2010 02:10 Edited 09-06-2010 02:21

Can you please explain why you can't exceed this termp, and what is the temp?
Tempering can be done at as low as 500F, which is why I asked if you did STRESS RELIEF at that temp.

3.2

By DaveBoyer (*****)

Date 09-06-2010 03:14

I don't know if He [Ron] does, but I can tell You WE did.

In repair welding tool steel parts that were tempered at 400f, We preheated at 400f, welded and then returned the parts to a 400f furnace for an hour, then let them cool.

By 3.2 Inspector (***)

Date 09-06-2010 03:33

This is all new to me....
If memory serves me well, you will need atleast 700 or 800 degees F in order to reduce the internal stress by 50%

Perhaps you are temper bead welding :)

3.2

By DaveBoyer (*****)

Date 09-06-2010 03:49

No, this is pretty standard practice for hardened & tempered tool steel weld repairs. If I knew then what I know now, I would suggest tempering again after the parts got to room temperature after the above procedure.

If You have not welded hardened & tempered tool steel, I don't doubt this is new to You.

By RonG

Date 09-06-2010 18:03 Edited 09-06-2010 18:17

All I can tell you (not knowing your requirements) is that if you exceed the tempering temp you have changed the hardness thus the yield and tensil.

Yes you can re-establish it by quench & temper but unless the weld filler material is hardenable it will loose the "As welded" strength but gain in some other areas like corrosion resistance.

Most tool steel (or most any high grade steel) has been quenched at 1550'F to 1800'F, allowed to cool (ambient) then tempered (drawn) to the required hardness. The higher the temper temp the softer the material.

In most carbon steel there is no metalorgical (spell correction needed) change below 600'F. Depending on the quench temp 500'F would have little effect but it does effect the grain structure enough to get more uniformity.

You can achieve stress relief (thermal) at or around 600'F but depending on the mass it could take an eternity (in production terms). You can also stress relieve by non thermal techniques like cryogenics and vibration.

Martinsetic material is mostly just quenched to get max strentgh.

By 3.2 Inspector (***)

Date 09-07-2010 01:45

Could you inform us the tempering temperature please?
A good ol' CS would be fine as an example.

3.2

By RonG

Date 09-07-2010 14:14 Edited 09-07-2010 14:20

AISI 4140 Steel:

Heat to 1525'F - 1600'F. Soak for 1 hour per 1 inch max cross section.

Quench in oil to below 150'F

Temper temps in F & hardness in RC scale
26-30 @ 1200'F =/- 25'
28-32 @ 1125'F
30-34 @ 1075'F
on down (temp) or up (RC scale) to-
40-44 @ 700'F
42-46 @ 625'F

4140 quenched at a high 1600'F will be darn near 50 RC.

Hope this helps
Foot note: Soak time as well as quench time are crucial to how uniform the hardness is thru the material.

By 3.2 Inspector (***)

Date 09-03-2010 18:10

Some scary reading on a Friday night....
Hope your **** gets rejected!

2.3

By MBSims

Date 09-05-2010 13:49 Edited 09-05-2010 15:11

From ASME IX:

postheating — the application of heat to an assembly
after welding, brazing, soldering, thermal spraying, or
thermal cutting

postweld heat treatment—any heat treatment subsequent
to welding

As others have said, postheating of welds is generally thought of as a hydrogen bakeout after welding which is usually done at temperatures between 300 F and 600 F, although it could be used for other reasons and at other temperatures. Postweld heat treatment is usually be applied for the purposes of relieving residual stresses, tempering heat affected zones, solution annealing to restore corrosion resistance, or can also be applied for other reasons. Postheating is not generally thought of as a "heat treatment", only the activity of applying heat.

Edit: If postheating were specified on a WPS, it would be typical that the postheating temperature range, hold time and method be specified in order to achieve the desired results.

By RonG

Date 09-05-2010 20:29

YEA! What MBSims said, but I would like to add.

I defer Heat Treating as: Setting a condition as specified by a Material spec. As often as not it requires 2 cycles "Quench & Temper".

Stress relief: Commercial heat treated 4130 (we use a lot) requires Stress relief after rough machining prior to finish machining. It's not a PWHT nor Heat Treat. Just a Stress Relief of stress's from the machining process much like a PWHT.

Most our WPS have a PWHT built into them because the will be finished machined to extreamly fine tolorances plus Stress Corrosion Cracking is a major concern.