4140 vs 4340 1

[robsb123]

My firm is planning on welding some pretty thick sections of alloy steel and I wanted to get some opinions on the process. We are planning to weld 5 1/2" thick by 8" wide forged ring with an ID of 7'-6" to the outside of a cylinder. The forged ring will be alloy steel (either 4140 or 4340) which will be annealed and machined to the dimensions given. The ring will also have a 3/4"x3/4" bevel on the inside for a groove weld to the cylinder (it will be a 3/4" groove and a 3/4" fillet on each side. The cylinder will be a rolled plate. The plate will be 60" wide and 1" thick A36 material.

After welding we plan to harden the OD of the ring since it will be riding steel on steel and will experience signifigant wear.

My questions are as follows:
1) What is the recommended filler metal and shielding gas (we plan to use FCAW) for each of these steels? (4140 vs 4340)
2) What is the recommended preheat and PWHT?
3) What is the recommended hardening process for this ring (I assume induction or flame) and what is a reasonable level of hardness for each material? (I do not need through hardness)
4) What precautions should I take in welding HSLA steel to low carbon structural steel?

Thank you in advance...

 

[metengr]

robsb123;

Here are responses to your questions;

1) What is the recommended filler metal and shielding gas (we plan to use FCAW) for each of these steels? (4140 vs 4340)

After reading your post, it would appear to me that a carbon steel filler metal would suffice because you are using the low alloy 4340 or 4140 to obtain the necessary surface hardness for wear, not strength. So, I would use
an SFA 5.20 E70T-6 electrode. This electrode is for horizontal and flat welding positions, no external shielding gas is required, polarity is DCEP, and is specified with an impact value of 20 ft-lbs at -20 deg F (review AWS 5.20 in detail for all of this information).

2) What is the recommended preheat and PWHT?

The preheat should be 350 deg F minimum for 15 minutes at temperature before welding. PWHT should be 1250 deg F for 1 hour at temperature. PWHT with electric resistance heaters with the cylinder in the vertical up position to avoid distortion.

3) What is the recommended hardening process for this ring (I assume induction or flame) and what is a reasonable level of hardness for each material? (I do not need through hardness)

I would use induction hardening after welding AND PWHT. It is fast, and is a localized heating method for this type of application. If you need surface hardness, I would shoot for 50-55 HRc if mated against steel. This is a guess, you need to make this call.

4) What precautions should I take in welding HSLA steel to low carbon structural steel?

Follow preheat requirements to the letter and use low hydrogen electrodes (H4 or H8 minimum). After the welding and PWHT are completed, I would highly recommend a wet fluorescent MT of the groove welds and surrounding base materials. Hire the NDT services out to a qualified organization.

 

[robsb123]

Thank you Metengr that post was very helpful. What is your resource for this information? Is there a reference that I can buy, or is it just years of experience?

How soon after welding do I need to complete the PWHT? Is the PWHT just a stress relief?

This is an outdoor application. The cylinder will be painted but the ring will be left bare. Does the 4340 have any better corrosion resistance due to it's nickel content?

You are correct in your assumption that we are choosing an alloy steel for it's wear properties and not it's strength. Since we are only looking for surface hardness then is there any benefit to using 4340 over 4140. How deep can I obtain 50 HRc in 4140? Using the 4140 will save us around $9000, which would be nice.

 

[metengr]

What is your resource for this information?

ASM Handbooks and AWS.

Is there a reference that I can buy, or is it just years of experience?

How much weld engineering do you do on a daily basis? Sometimes it is better to pay for this specific expertise. If you really want to delve into welding, go to the ASM International web site and AWS.org web site.

How soon after welding do I need to complete the PWHT? Is the PWHT just a stress relief?

As long as you used low hydrogen electrodes you can allow the weld joints to stay at room temperature for an indefinite period of time before PWHT.

No, the post weld heat treatment I mentioned is to reduce residual stress and to soften the low alloy steel base metal heat affected zone.

This is an outdoor application. The cylinder will be painted but the ring will be left bare. Does the 4340 have any better corrosion resistance due to it's nickel content?

No.

You are correct in your assumption that we are choosing an alloy steel for it's wear properties and not it's strength. Since we are only looking for surface hardness then is there any benefit to using 4340 over 4140. How deep can I obtain 50 HRc in 4140? Using the 4140 will save us around $9000, which would be nice.

For this application, 4140 alloy steel would work. The 4340 alloy steel is used for deeper hardening capability and toughness. In your case, I would go with 4140. For induction heat treating in the field, I would contact a Mannings or other induction heat treatment company that can do this work in the field.

 

[robsb123]

Metengr,
I do not do that much weld engineering on a daily basis. I am relatively young (only three years out of school with a BSME). I am working with a company that does a significant amount of structural steel welding. I would like to get a little deeper into material selection beyond the standard structural grades. I would like to know more about material selection and welding. I deal mostly with carbon steel, some stainless (almost always 304), and very little aluminum.

I bought Metallurgy for the Non-Metallurgist and thought that it was rather vague. I would like a handbook that goes through each material and gives information such as weldability, machinability, hardenability, ease of fabrication, and gives comparisions between materials. I am primarily focused on low and medium carbon steels and HSLA steels.

What resources can you recommend for a young and eager engineer?

How much is an ASM membership?

I appreciate all of your help, Metengr. I regularly scan the latest posts on this forum, and you always seem to have a very imformative answer.

Thanks,

 

[metengr]

robsb123;
Great. I would suggest that you purchase ASM Handbook, Volume 6 on Welding, Brazing and Soldering. You can also look ar several other Volumes (1 and 4). This is probably one of the most comprehensive books that will provide you with information that addresses all of your needs. Also, you should review AWS D.1.1 and also AWS B2.1. These would be helpful for your structural steel work.

http://www.asminternational.org/

Go the the bookstore tab and shop around.

 

[robsb123]

This will be my last round of questions on this subject... I think.

We are beveling the ring at a 45 degree angle 3/4" deep in each side. We plan to fill the groove and reinforce with a 3/4" fillet.

What wire size, and corresponding voltage/amperage, do you recommend?

How many passes to complete this weld?

What is recommended for a maximum interpass temperature?

What method do you recommend for measuring preheat and interpass temp?

Okay, I'm finished...

 

[dimjim]

You will probably get greater depths of hardness with the 4140 material. The 4340 will give you greater ductility.
I do not know which is more important to you.

 

[robsb123]

Really, I thought that 4340 was typically used for larger parts, because it was better for through hardening. What type of depth could you expect for each material at 50 HRc?

 

[metengr]

We are beveling the ring at a 45 degree angle 3/4" deep in each side. We plan to fill the groove and reinforce with a 3/4" fillet.


What wire size, and corresponding voltage/amperage, do you recommend?

Ans. Review AWS 5.20. Also, I would urge you to qualify a welding procedure for this job. Hire out the services of a CWI for guidance and council. I don't believe you need to spend the $$ on a Consulting Engineer.

How many passes to complete this weld?

Ans., It depends on the wire diameter.

What is recommended for a maximum interpass temperature?

Ans., I would use a maximum of 650 deg F.

What method do you recommend for measuring preheat and interpass temp?

Contact pyrometers for preheat maintenance and thermocouples for monitoring PWHT. I would suggest using all electric resistance heaters for preheat maintenance and post weld heat treatment maintenance.

 

[metengr]

Regarding depth of hardening with a 4140 alloy steel (austenitized at 1600 deg F), the hardenability data I have from Timken indicates hitting 50 HRc at 10/16" in depth.

In comparison, 4340 will easily hit 55HRc at 10/16" depth.

 

[dimjim]

Meteng,
My Timken Manual shows the same as yours.
I was positive that the shop was telling us
to back off on the depths of hardness of 50Rc
but the charts show the 4340 superior. Hmmm.
Thanks for calling that to my attention.
I was thinking that the nickel content made it
respond more like 8660.

 

[NoSpatterWeldEng]

The ASM Vol. 6 doesn't have a recommendation for this thickness but they list 2" thickness preheat and interpass temperatures. 4140 is 550-650 deg. F and 4340 is 600-700 deg. F. I wouldn't recommend going over 900 deg. F based on my experience with those materials but I've never done anything that thick. Almost always: greater thicknesses require higher preheat temps.

From experience I've seen great results at 900 deg. F but note that is also the typical tempering temperature for 4140 after quenching.

I know you probaly will use (or have used) a low strength filler metal but the preheat temperature is really for the low alloy material crack resistance.

 

[metengr]

NoSpatterWeldEng;
Sorry, but I would have to disagree. You can calculate the CE (carbon equivalent) for 4140 low alloy steel, which is around 0.78. For most of the 4140 steels I have dealt with that are 2"-4" in diameter, I have used low hydrogen electrodes with a minimum preheat of 350 deg F (adequate soak time before welding). The 550 deg F-650 deg F sounds more like a maximum interpass temperature range, not a preheat temperature.

For 4340 alloy steel, again using low hydrogen electrodes, I have had great success at 400 deg F preheat. This is the temperature where hydrogen is readily diffusible, so I really can't see going much above this temperature. It really serves no purpose other than to waste energy. More heat does not always imply trouble free welding.

 

[EdDanzer]

Having welded a lot of 4140 to A514 plate in a high impact application I would stay above 300°f and make sure you are closer to 500°f when welding. Low alloy filler materials at lower preheat temperatures may not mix as well and create middle of the weld cracks when stressed.