Welding 4140

That sounds pretty close. I presume you're welding that in the annealed condition and then heat treating the completed weldment to the desired physical properties? I'd keep the preheat in the upper end of that range, especially in the thicker sections. Your filler metal will want to be in the range such that it has similar heat treating behavior as the base metal. I also presume that the completed welds are meeting specification and NDT requirements? If they are, then you must be on the right track.

I have few observations. to make.

1. The filler metal selection is OK.
2. Preheating temp. is more than sufficient
3. Post weld HT temp for these kind of steels are generally 1100 to 1360 ⁰ F ( 590 -740 ⁰ C ) with min.cooling time of 1 hour per inch thick and air cooled only after 600 ⁰F ( 325⁰C ) The low holding cycle provided may not help to remove full stresses formed by heat during welding. Proper cycle will help and result in reduced hardness, residual stress level, increased ductility of the weld metal and HAZ.
4. Hence check the hardness values at weld and HAZ are as the un-released stresses may create under bead cracking either during or after welding due to thickness factor.
5. It was not mentioned whether the job undergoes high temp. cyclic load or at normal temp applications.
6. If you are using extra low carbon filler metal ( C – 0,03 % ) i.e., ER80S-D2L, the filler wire will have enough as-welded ductility to meet the code requirements. Post heating may therefore be avoidable in some cases.
7. As far as possible Cr-Mo steels must be pre heated, welded and post weld heated in one continuous operation to avoid under bead cracking.

If it is OK and possible the same above welding can be carried out using ER 312L, ER 309 L,

ER 316 L ( 1.2 or 1.6 mm size) type austenitic stainless steel filler wires also where post weld heat treatment can be avoidable. Though yield is less comparatively, this extremely ductile metal may resist cracking due to shrinkage stresses.

There are many success stories with use of SS filler wire provided,

1. The joint does not undergo cyclic temp. in service.

2. The difference in coefficient of thermal expansion between the weld and base metal does not give raise to stresses at high (or) at cyclic temp in service.

If required U can qualify the Procedure and do transverse tensile, bend tests, Micro analysis to check the metallurgical factors before putting into operation.

Welding with austenitic weld metal is more common in the Petroleum Industries where more or les study temperature exists than in Power plants where temperature and pressure fluctuates

C Sridhar, advance institute of welding technology, Chennai –India.

Thank you, very informative and helpful. For clarification, the part operates at normal temperature and an austenetic filler is not needed. Also, I am under the impression that the PWHT (450F-650F) is to reduce embrittlement in the HAZ. However, we have had several weld issues and about 70% of test samples have shown weld defects.

I am new to the company and after watching the process I discovered several areas where these weld defects might occur. First, the joint is a V-groove circrumferential weld joint,(multiple pass) and continuous. Issues I see- Operator is adjusting both weave and travel speeds (in progress, at the completion of each pass) allowing a window where defects can occur. The positioner motor turns at a slow speed and the joint can only be welded in short-circuit. This mode of tranfer is notorious for its susceptability to lack of fusion (which has been the case in the root on the majority of samples).

A few more questions:

A higher PWHT will result in more deformation, correct? Is there anyway to avoid this?

What do you mean by "underbead cracking due to thickness factor"? Residual stress caused by more weld?

What code specifies the welding for this material? We are not currently welding to any code, although we do perform destructive testing and mettalurgical analysis.

I was also under the impression that it is best to match filler metal and base material strength and composition. Why would we be not using a 110 or 120 ksi filler,(obviously weld strength may not be a factor) what are the advantages, or disadvantages of using a higher strength filler metal.

Must the PWHT be at the range you specified to prevent underbead cracking?

Can you define what you mean by cooling a little better? Process?

Again, I appreciate your comments and expertise in the area.

Thank you, very informative and helpful. For clarification, the part operates at normal temperature and an austenitic filler is not needed. Also, I am under the impression that the PWHT (450F-650F) is to reduce embrittlement in the HAZ. However, we have had several weld issues and about 70% of test samples have shown weld defects.

You have not mentioned types of defects you have come across. I assume it could be LF ( due to thickness and improper weaving factors) or cracks due to hydrogen ( cold cracks) or faster cooling rate in your 1 to 1½" thickness of the parent metal. But all are avoidable.

I am new to the company and after watching the process I discovered several areas where these weld defects might occur. First, the joint is a V-groove circrumferential weld joint,(multiple pass) and continuous. Issues I see- Operator is adjusting both weave and travel speeds (in progress, at the completion of each pass) allowing a window where defects can occur. The positioner motor turns at a slow speed and the joint can only be welded in short-circuit. This mode of tranfer is notorious for its susceptability to lack of fusion (which has been the case in the root on the majority of samples).

Once the parameters including amps. Voltage, travel (rotation) speeds are fixed, operators should not be allowed to tamper with the settings . That will create lot of unknown complications during the welding but, resulting in defects after its completion.

You can try with Pulsed MIG Process, in which one can use spray transfer technique . Alternatively inverter MIG power source. Pulse MIG compared to convention MIG helps to avoid short circuit transfer and increases your Positioner rotation speed . This may help to avoid LF once for all. Though you have to use Argon, it is still economical due to higher productivity and lower rejection / defect rates.

A few more questions:

A higher PWHT will result in more deformation, correct? Is there anyway to avoid this?

What you are doing is Post Weld Heating, which as you pointed out is to allow the dissolved hydrogen to escape before the weldment has cooled down to below 150 ⁰C ( 300⁰ F) the temperature at which Hydrogen Induced Cracking (HIC) begins to occur.

Since U said it is PWHT, I felt is Post Weld Heat Treatment a kind of stress relieving operation done at higher temperature to remove the residual stresses formed due to heat input at the time of welding . For your job it may not be required.

What do you mean by "underbead cracking due to thickness factor"? Residual stress caused by more weld?

Under bead cracking is cracks located at HAZ underneath the weld bead. It is nothing but, cold , delayed or Hydrogen induced cracks. In MAG process it may occur due moisture in CO₂ gas (or) in the atmosphere. Cracks are seen after 20 hours or so after welding .

This is nothing to do with residual Stresses where cracks ( due to thickness also) which will be visible immediately after welding.

What code specifies the welding for this material? We are not currently welding to any code, although we do perform destructive testing and mettalurgical analysis.

You can follow ASME Sec IX for your WPS, PQR and WPQ and fix the parameters once for all. All DT and NDT techniques will figure in it.

I was also under the impression that it is best to match filler metal and base material strength and composition. Why would we be not using a 110 or 120 ksi filler,(obviously weld strength may not be a factor) what are the advantages, or disadvantages of using a higher strength filler metal.

If the weld strength is a factor then what you suggested is OK. Since it is not the case , you selected NI-MO based filler wire. High strength filler metals has lower ductility and susceptible to cracking if proper procedure is not followed. On the other hand higher ductile filler wires has high resistance to cracking and has more life.

That 's why at times we use SS filler wire which has more than 35 – 40 % ductility and more UTS compared to base metal and avoids pre heat and post heating also.

Must the PWHT be at the range you specified to prevent underbead cracking?

Not required. If proper procedure is followed and as long as no cracks are seen. It is again more to do with pre heating than post heating.

Can you define what you mean by cooling a little better? Process?

I am sorry. It should be soaking at a particular temperature and not cooling --- by over sight..

Trust above details are useful to U.

C Sridhar

Thank you, very useful and I appreciate the response.