Let’s keep it simple. 4140 steel can be welded without problems as long as you follow some simple guidelines. This also applies to AISI 4140 and 4340.  We will not be discussing the metallurgical properties of 4140 or trying to explain what happens to its microstructure when it cools too rapidly, just remember that the key is to control our cooling rate. If the weld and heat affected zone cool too quickly we are in trouble.

AISI  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 4140, or other medium to high carbon steels for that matter, as compared to low carbon steel.

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 60F [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 fabrication if 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 (formation of martensite and hardening of the weld and HAZ), 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 [205 – 260C]

1/2″ – 2″ thickness  600-700F [316 – 371C]

Other  publications recommend a preheat of only 350F [177C] up to 1/2″, 450F [232C] from 1/2″ to 1″, and 500F [260C] for 1″ to 2″.  Keep in mind that when a part is preheated over 500F [260C] 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 be kept at or above the preheat temperature.  If more than one pass is required make sure the part is above that minimum interpass temperature, if not, apply more heat to the part with oxyfuel or other means.

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.