How to weld AISI 4140 Steel

AISI 4140 is high-strength medium carbon steel. 4140 can be easily welded with the right precautionary steps being applied. Other steels in this category as AISI 4130 and 4340 follow the same welding advice. This article provides a detailed introduction to the AISI 4140 materials, their properties, welding, and PWHT requirements.

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What is AISI 4140 steel?

AISI 4140 is a Cr-Mo-Mn-based high strength low alloy steel offering high fatigue strength, toughness, abrasion & impact resistance. Being a medium carbon steel range, AISI 4140 gets hardened during welding repair especially in the heat-affected zones (HAZ) due to thermal cycles and microstructural transformation along with locked residual stresses produced by the welding heat. This results in the formation of bainite and martensite depending on the cooling rates. The high hardness in the HAZ is associated with the locked residual stresses and the martensite transformation of the AISI 4140 microstructure.

Material Specification for supply for AISI 4140

ASTM A322, ASTM A331, ASTM A506, ASTM A513, ASTM A519, ASTM A646, ASTMA752, SAE J404, SAE J412, SAE J1397 etc.

Chemical properties of AISI 4140

The table below shows the typical chemical properties of AISI 4140 steel material in as supplied conditions. The material is usually available in quenched and tempered condition and designated as AISI 4140HT. Although it’s available in annealed condition too.

   Carbon       Si     Mn       Cr       Mo        S      P
      0.40     0.30      0.88      1.10      0.24     0.015    0.021

Weldability of AISI 4140

Being Low alloy steel along with high carbon content & high carbon equivalent (C.E.) of around 0.8, the welding becomes a challenge due to the propensity of high hardness in the heat-affected zone and weld metal resulting in weld cracking if the right precautions are not considered. The figures show the typical microstructure of AISI 4140 material and weld metal.

How to weld AISI 4140

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Selection of Welding Consumables:

The selection of filler wire is based on the chemical and mechanical properties of the AISI 4140 material. The best filler wire or electrode is E8018-B2 as SMAW electrode or ER80S-B2 when welding with TIG or GMAW. The table below shows the typical chemical properties of ER80S-B2 filler wire.

  Carbon     Si    Mn    Cr     Mo   Ni      S      P
0.07- 0.120.4- 0.700.4- 0.70 1.2- 1.500.40- 0.65  0.20    0.025    0.025

The AISI 4140 can also be successfully welded using E7018, ER70S-2 (undermatching consumables), or other most common carbon steel welding consumables although they result in softer weld metal as well as reduced mechanical properties of the weld compared to AISI 4140.


AISI 4140 steel must be preheated to avoid the:

  1. Formation of hard microstructures such as martensite and Bainite in the weld and HAZ.
  2. Allow weld to slow cool thus help to release residual stresses.
  3. Allow the release of weld metal hydrogen.

The AISI 4140 steel is recommended to preheat:

  1. Up to ½ inch (12 mm) thickness: 400- 500℉ (200- 250℃ approx.)
  2. Above ½ inch (25 mm) thickness: 500- 600℉ (250- 300℃ approx.)

The above preheat temperature is based on the recommendation given in Welding handbooks such as ASM and AWS.

You can use our Online Welding Preheat Calculator to find the right preheat temperature for the variety of materials based on alloy chemistry, plate thickness, and hydrogen level.

Watch our YouTube video to Learn what is Preheat, Inter-pass & post heating.

Interpass Temperature:

Usually, the interpass temperature is kept above the minimum preheat temperature. In no case, the interpass temperature shall be allowed to cross the tempering temperature of the supplied materials as it will damage the material properties.

After Weld Finish:

Allow the welding area to slow cool using a heating blanket is a good practice. It will prevent fast cooling thus prevent any weld cracking, support soft microstructure and remove hydrogen from the weld.

Post Weld heat Treatment:

Post weld heat treatment shall be carried out 1225 to 1275℉ (660- 690℃). In case the material is quench, tempered (Q & T), the PWHT temperature shall be below the tempering temperature usually  50 to 60℉ (10- 15℃) below it.

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