How To Weld 4140 Steel

Let’s keep it simple. 4140 steel can be welded without problems as long as you follow some simple guidelines. We will not be discussing the metallurgical properties of 4140 or trying to explain what happens to its microstructure when it cools too rapidly.  4140 steel is alloyed with chromium and molybdenum, hence the term chromoly steel.  The addition of these alloying elements and its relatively high carbon content (around 0.40%) provide this low-alloy steel with excellent strength to weight ratio and make it a lot stronger and harder than standard 1018 and 1020 steels.  The disadvantage is the need to be more careful when welding 414, or other medium to high carbon steels for that matter, as compared to low carbon steel.

When welding 4140 material it is absolutely necessary to preheat.

When welding 4140 material it is absolutely necessary to preheat.

4140 is typically supplied in either the hardened and tempered condition (referred to as 4140HT) or in the annealed condition.  For welding it is recommended that you use the latter.  Welding on 4140HT is not recommended due to the high susceptibility for cracking, even when using preheat.  If you must weld 4140HT it is absolutely necessary that you stress relieve the part at a temperature 15C below the original tempering temperature.  Typically, if you have to weld on 4140HT you are making a repair.  4140HT should never be used for standard fabricationif possible.

If you are manufacturing parts out of 4140 we can assume that it is in the annealed condition.  4140 will weld very similar to your low carbon steels.  The  difference is that its high carbon content can screw things up.  To avoid cracking you need to preheat 4140 prior to welding.  This is done to slow the cooling rate after we are done welding and avoid embrittlement, which can lead to cracking.

Steps to weld 4140:

1. Preheat

Per Table 6-13 in page 6.1-29 of The Procedure Handbook of Arc Welding we need to preheat as follows:

Up 1/2″ thickness   400-500F

1/2″ – 2″ thickness  600-700F

Other  publications recommend a preheat of only 350F up to 1/2″, 450F from 1/2″ to 1″, and 500F for 1″ to 2″.  Keep in mind that when a part is preheated over 500F  it becomes extremely uncomfortable for the welder.  These lower preheat temperature are used with success, but go with the higher ones is you want to play it extra safe.  The higher temperatures will produce slower cooling rates.

It is important that the interpass temperature not be below the minimum preheat temperature.  If more than one pass is required make sure the part is above that minimum temperature, if not, get that rose bud and heat the part.

2. Use a low-hydrogen electrode (whether using GTAW, GMAW or SMAW)

The filler metal recommended for welding 4140 steel is ER80S-D2.  ER70S-2 can be used but you will be undermatching the base material.  Low-hydrogen electrodes minimize the amount of diffusible hydrogen in the weld.  With the elevated carbon content even low levels of hydrogen can wreck havoc. A common problem with medium to high carbon steels is their susceptibility to hydrogen-induced cracking.

3. Slow cool the part. Preheating  will take care of this most times; however, using heating blankets to assure a slow cooling rate is good practice.   As mentioned above, a slow cooling rate prevents or reduces embrittlement (crack sensitive area in the heat affected zone).  It also allows hydrogen more time to leave the weld, thus reducing the possibility of hydrogen-induced cracking.

Do you typically weld 4140  or other high  carbon steel? Please include your thoughts and any tips in welding these materials.

 

 

Please note: I reserve the right to delete comments that are offensive or off-topic.

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36 thoughts on “How To Weld 4140 Steel

  1. If I weld AISI 4140 with AS 3678 Gr350, what consumable I have to use for this combination and what is the preheat nad postheat temperature?

    • Vijai, AS3678 has 22 points of carbon max, 4140 has approximately 4140,so definitely need to consider preheat. However, this depends on the the thickness of the material. Can you please provide more details? Material thickness, joint type. ER80S-D2 is typically OK, but may not be optimal depending on the application.

  2. I am trying to weld Lift lug (25 mm thick plate -AS3678 Gr350) to clump weight (725 mm dia x 725 mm height – AISI 4130 / AISI 4140). Pls advise preheat temp., weld process, type of filler to be used, PWHT reqd or not, if PWHT required than at what temp / soaking time / cooling process.

    Thanking you

    • Hi Rusty, to provide a more complete recommendation we need a lot more information (application/intended purpose, service conditions, joint type, etc). For now here are a few tips. First, do you need to match the strength of the 4140 casting with the weld? Otherwise you can use an ER70S-6 GMAW wire. Preheat 600-700F and slow cool. The need for PWHT depends on the final mechanical properties you need.

  3. Hi,

    I am trying to plug a 6mm (0.236 inches) dia hole in a tool (to repair the tool) that will be used in oil and gas application and will experience pressures upto 1378 bar (20 000 psi) at high temperatures of about 100 degree celsius. I was wondering if welding the hole will seal is off effectively and plug the hole permanently. Thanks in advance.

    • Properly welding the hole can definitely provide a good seal. However, care must be taken. How deep is the hole? If it is too deep it may be difficult to achieve good side wall fusion. In some cases,if too deep, you may need to open up the how a bit. Also, what is the base material and how thick? Depending on the amount of carbon and thickness you may need to preheat in order to avoid excessive hardening in the heat affected zone. Also, make sure the surfaces are very clean as contaminants such as mill scale can prevent good side wall fusion.

      • Hi welding Guru,

        I would like to run a PQR for AISI 4145 to AISI 4145. kindly advise me which electrode can be used to weld this PQR so that it meets the minimum tensile strength requirement and Bend test requirement as per ASME Sec IX. This material is being used as per API 5D for drilling tools. The actual scenario is that weld build up / buttering to be done after the removal of existing hard banding on a drill tool.

        • AISI 4145 is considered an unlisted material so it has not group number assigned. Qualification will need to be done by testing. The UTS of AISI is 135ksi. The question is: do you need to match the tensile strength of the base metal for this application or are you using this material for its hardness. This is typically the case for tools that have to resist abrasion. AISI 4145 is used extensively on down hole drilling tools such as drill collars due to its high carbon content which in turns provides excellent hardenability.
          Since you will have to qualify by testing it is important to understand the requirements. In many cases similar materials are welded with 110Ksi filler and with dilution the test results creep into the 120’s.

  4. dear Gentlemen,

    kindly i am asking about what is the best welding electrode to weld Hardox600 with steel 4140

    • Mohamed, I will email you a guide to the eamil address you provided. This is a welding guide published by SSAB which is the manufacturer of Hardox wear plate. Selection of the right filler metal is important. Equally as important is strictly obeying the minimum and maximum preheat and interpass temperature. Welding Hardox can be done without any issues. If the weld does not need to match the tensile properties you can weld with the GMAW process using an ER70S-6 filler metal. This is true for both 4140 and Hardox 600. You can also us an ER80S-D2 wire. This guide should point you in the right direction. If you do not receive it by email you can find it on SSAB’s site at:
      https://www.ssab.com/products/brands/hardox/products/hardox-600#!accordion=workshop
      under Workshop and Design Information.

  5. I’m making an angle plate for machine shop use. I’d like to add a brace, as most angle plates have them. I’m using 4140 as I have some around, and the hardness is helpful for wear resistance. The strength isn’t particularly important.

    It’ll be around 7/8″ thick when welded. I’m not sure what I’ll use for the brace, maybe more 4140, maybe 1018 or similar, not sure on thickness either – 1/2″ minimum, but I might go thicker.

    This is all being done in a home shop, and I don’t have a torch big enough to reasonably preheat. The best option that I can think of is to tack it up, throw it in my oven (which I think is good for a little over 500F) for a while, run a single 7018 bead down each side using 1/8″ rod (I don’t have a machine that’ll run bigger), and bury it in dry sand.

    After welding I’ll machine it to final dimensions (3/4″ thick for the L shape part).

    If I get it up to 500F, how likely is it to crack?

    How about if I just weld it at room temperature, is cracking likely in that case?

    Thanks for any advice.

    • Hello Daniel, the process you are describing sounds good. It’s better than how it is being welded in some shops we visit. Preheating to 400-500F with your oven, welding with 7018 then slow cooling by burying in sand should take care of problems associated with cracking due to embrittlement in the HAZ. The reason we preheat is to have a very slow cooling rate which avoids, or minimizes, the formation of martensite. See the post on hydrogen induced cracking for more information: Preventing Hydrogen Induced Cracking and Factors Influencing Hydrogen Induced Cracking.
      We do not recommend welding without preheat, but it is theoretically possible if you use an extremely high heat input when welding, typically this can only be achieved with submerged arc welding. You would be nowhere near with a 1/8″stick electrode.
      The E7018 is a good choice for filler in this case. We would suggest using A1018 for the braces as you mentioned. Good luck.

  6. Hi, I am welding AISI4140 block to a 6mm thickness ASTM A588 rectangular hollow. May I know what kind of electrode shall I use? And is that OK if I preheat it to 500F?

    Thank you.

    • ASTM A588 is a Group II base metal according to AWS D1.1. Filler metal for matching strength would be any low hydrogen stick electrode (E7015, E7016, E7018, E7028), any F7XX flux-cored electrode. For GMAW use ER70S-X (ER70S-3 or -6) electrode. GTAW use ER70S-2. 500F preheat should be plenty. Recommended preheat is 400-500F for AISI 4140 up to 12mm in thickness. Are there any special requirement? What is the application? Answers to these questions may allows us to be more specific with process and filler metal.

  7. I need to arc weld a piece of 4140 that has 4″x 4″x1/8″ deep hammered marks in it that needs welded and machined flat , should I preheat this piece its 4″ thick by 36″ in dia. I was just going to use a machineable rod for this 7018MR

    • You definitely need to preheat. For 4″ thick material go at least 650-700F if possible. Make sure your interpass temperature is at least 650F as well. If you cannot get it that hot try to slow the cooling rate down by throwing a heat blanket or bury in sand. Fast cooling rates may cause cracking. Even though this is for build up and not joining the same rules still apply. Low-hydrogen electrodes are necessary so 7018MR should work. Are there any other specific requirements for this deposit?

  8. WE ARE GOING FOR CONDUCT A PROCEDURE FOR AISI 4145 TO AISI 4140 ( L80) AND 4145 TO A36/ 4140 TO A36. WHICH WELDING ELECTRODE WE CAN USE FOR THIS PROCESS.
    E10018 OR E12018-H is Possible Please advise.
    thanks u

    • What are the requirements of the weld in terms of mechanical properties (tensile/yield strength, elongation, etc)? This affects filler metal selection. AISI 4145 has a tensile strength ranging from 94 to 128 Ksi (650 – 880 MPa). In most cases you don’t need to match the strength. If you are welding 4145 to 4145 and needed to match the strength of the base material then E12018-H would be a good choice. Remember to preheat and slow cool adequately to avoid cracking.

      When joining to metals with lower tensile strength (like 4140 and A36) you only need to match the strength of the weaker base metal. So welding A36 to 4140 can be done with a E7018. For 4145 to 4140 you can use E12018, but keep in mind 4140 may have a tensile strength as low 94 Ksi, in which case E10018 would suffice. You need to know if the material is cold drawn or if it has been heat treated.

  9. hi dear please advise about PWHT, We are going conducting PQR 4145 X4140 PIPE, 4145X A36 AND 4140X A36 its all required pwht on the test coupon waht si temp.
    please advise.

    • When performing the Procedure Qualification Record you should use preheat and PWHT if that is what the base material needs. In this case you do need to preheat and PWHT according to what you expect to run in production. Remember that the qualification process is making sure that what will be done in production works, so your WPS must be based on the procedure used for your PQR. 4140 and 4145 typically don’t require PWHT other than simply slow cooling, but this may depend on the final properties you want out of your welded material.
      The Procedure Handbook for Arc Welding suggests using a preheat of 400-500F for thickness up to 1/2 inch and 600-700F for thickness of 1/2 inch to 2 inches.
      If this weld requires multiple passes make sure your interpass temperature is the same (minimum) as your preheat.

    • What is the thickness of each of these materials? What welding process will you be using (GMAW,GTAW,SMAW,etc.)? 1045 and 4140 will definitely need preheat and slow cooling, but the actual temperature depends on material thickness.

    • Both Hardox 500 and ASTM A514 will require preheat. You need to know what the weld requirements are. If you do not need to match the strength of the base metals then don’t. Both of these base metals are prone to hydrogen induced cracking. Reduce this risk by:

      * Minimizing the hydrogen content in and around
      the prepared joint
      * Use the right preheat and interpass temperature (this will depend on the thickness of the material and could also be affected by the the requirements of the A514 in terms of harndness)
      * Use welding consumables with a low hydrogen content
      * Keep impurities out of the weld area

      For additional information on hydrogen induced cracking read: Factors Influencing Hydrogen Induced Cracking and Preventing Hydrogen Induced Cracking.

  10. Hello,

    Great information!

    I am looking to create a strong and cosmetically attractive weld joining a forged hammer head to a forged handle. The material is 4140. I will ultimately want to automate the weld process, but for the first articles I am planning to hand weld in order to fully understand the process that creates the best weld result. I will be post heat treating the assemblies to eliminate stresses from forging and to force a RC40-45 hardness. Question is, given the post heat treatment, must I pre-heat the components prior to welding? If so, do you have any recommendations for the best way to achieve this? I am thinking a hand torch with temp indicating chalk until the process is measured, and then using timed torch application when automated.

    Also, would yo uall recommend MIG or TIG? I feel that MIG will be most straightforward for automating and easier for a novice like me to achieve a cosmetic result.

    Sorry for long winded question! I appreciate any and all replies.

    • Hi Ben, good question. Regardless of whether you will be doing any postweld heat treatement you will need to preheat the material. We preheat 4140 and other medium to high carbon steels to slow down the cooling rate. This in turn reduces the formation of martensite with which is very hard and brittle. The slow cooling rate also helps in reducing susceptibility ot hydrogen cracking.
      You can use different methods to preheat – put parts in an oven or furnace, use induction heating or use as torch as you describe (oxygen with propane or acetylen works great). We always want to measure the temperature at least 1 inche away from the joint in ALL directions. If the thickness of the material is over 1 inch then take the temperature reading at a distance equal to the thickness of the material. If you are welding on a small hammer it shouldn’t take long to preheat the entire part – this will certainly give you a very slow cooling rate and eliminate a lot of issues.

      Both MIG and TIG are good processes to use. TIG welding usually has a higher overall heat input due to how slow it is compared to MIG. Higher heat input means slower cooling rate. However, since you’ll be preheating this is not a concern. TIG will have a better cosmetic appearnce but you can certainly make great looking welds with MIG, especially if you end up automating the process. Hope this helps and good luck!

  11. Can a 4140 HT pipe test be successful if preheated to 400 degrees F with 17 to 19 volts for the root and 17 to 21 for the balance using GMAW .035 ER80SD2 wire. Interpass temperature 600 degrees F and 1 hour post heat of 1125 degrees F? Pipe is 1 inch thick with backing. 60 degrees V groove.

    • Hello Victor. Welding 4140HT is not recommended by the manufacturers of this steel and by many who have done it in the field. There are a few things to consider. When you weld on 4140HT you will be annealing the the material near the weld (HAZ), so your end product will not be what the base material is (different microstructure). You will have a significant variation in harndess and ductility. If you absolutely have to weld on 4140HT you need to do post weld heat treatment of the weldment to relieve the stresses as you mentioned in your comment along with the recommended preheat and interpass temperature based on material thickiness. Check with the manufacturer of the steel regarding the actual temperature range needed for stress relieving. Also, the cooling rate must be controlled. Keep in mind that if you have the ability to PWHT then there should be no reason why you can’t anneal the part prior to welding, and then PWHT to bring back to the hardened condition. Thus avoiding the high susceptibility to cracking when welding in the hardened condition.
      Like anything else, our recommendation is to qualify your welding procedure. You can weld and PWHT exactly as you describe. Perform tensile and harndess tests in weld metal, HAZ and beyond the HAZ as well as any other tests required by the code or standard you are following. Make sure the properties you require can be achieved with your procedure.