As one of the most common of the heat-treatable low-alloy steels, 4130 steel can be found in a number of industries and applications. It is also among the most likely to cause confusion when it comes to welding.
Its high hardenability is the key to 4130 steel responding to heat treatment. The material’s varying mechanical properties, however, can make it challenging to select a filler metal for welding that matches its strength in a given condition.
For that reason, there are many different ways to weld 4130 steel, depending on the application and the desired outcome. Fabricating thin wall tubing for a race car frame with 4130 steel, for example, has different requirements than using the material to fabricate heavy-duty, multi-inch valves for petrochemical applications.
This article provides some best practices for selecting the proper filler metal for welding 4130 heat-treatable low-alloy steel, along with recommendations for pre- and post-weld heating.
The 4130 steel alloy relies on the addition of carbon as well as molybdenum and chromium as agents that increase hardenability – the ability to increase hardness and strength through heat treatment. The strength of the material can change greatly depending on its condition: annealed, normalized, or quenched and tempered. For instance, the approximate tensile strength for annealed 4130 steel is 90 ksi while the material tensile strength in a normalized condition is around 100 ksi; quenched and tempered 4130 steel is stronger, offering tensile strength up to 200 ksi.
In addition, several types of filler metals can be used to successfully weld 4130 steel. While welding operators may assume they need a filler metal that matches or overmatches the mechanical properties, that is not always the case.
Consider these three options for selecting the appropriate filler metal based on the condition of the 4130 steel and the demands of the application.
Strength undermatch: When a part design or the application requirements do not require the weld deposit to match the strength of the 4130 steel, it is possible to use a lower strength filler metal, such as mild steel wire or stick electrode offering a tensile strength of approximately 70 ksi. Mild steel filler metals offer the advantage of additional crack resistance due to their ductility, but the resulting weld will not be as strong as the base material. As long as the part design allows for this undermatch, that lower strength weld should not pose problems. See Figure 1 for mild steel filler metals often used for strength undermatch when welding 4130 steel.
Matching annealed strength: If the 4130 steel is in the annealed condition (the recommended condition for welding to avoid cracking) and will not be post-weld heat treated to increase material strength, using a filler metal with comparable strength properties is common and acceptable. For an approximate match to annealed strength 4130 steel, low-alloy filler metals offering 80 ksi to 90 ksi tensile strength are appropriate. These filler metals provide the advantage of a strength match in the weld deposit without the crack sensitivity of a chemistry match. See Figure 2 for filler metals that can be used for this option.
Chemistry match: For 4130 steel that will be post-weld quenched and tempered or hardened, it is necessary to have a filler metal (and resulting weld deposit) that responds to the treatment in the same way the material does. In other words, match the 4130 steel with a 4130 filler metal providing the same strength and chemistry.
It is important to note, however, that there are high carbon levels in 4130 steel (which is what makes it heat treatable); as a result, it is more sensitive to cracking. Therefore, if the application does not require post-weld heat treatment, there is no advantage to using a chemistry match with 4130 filler metal.
Other filler metal options can be used to weld 4130 steel, such as a 309 austenitic stainless steel product. This type of filler metal offers good strength, is good for joining dissimilar materials and provides additional crack resistance. Be aware, this filler metal typically is also more expensive, and many jobs can be successfully completed with less costly, more readily available filler metals.
Preheat, slow cooling and stress relief are all precautions that can – individually or combined – reduce the risk of cracking when welding 4130 steel.
Preheat can be used to slow the cooling rate and minimize the development of crack-susceptible microstructures within the weld. The more heat that is in the base material before welding begins, the slower it will cool down. See Figure 3 for preheat recommendations.
Slow cooling of welded parts can be controlled with blankets, ovens or other methods. This practice offers the same advantages of preheat – to reduce the chance of unwanted microstructures forming, which helps the part resist hardening or cracking.
Post-weld stress relief is another method to help prevent cracking and defects in the finished weld. Thin materials, less than 1/8 in., typically do not require stress relief because cracking is less of a concern. Thicker materials are commonly stress-relieved at 1,100 to 1,250-degrees F for approximately one hour per inch of base material thickness.
Keys to success
As with the welding of any material, knowing the basics about filler metal selection and material properties can help make welding 4130 steel a success. The key is to know what condition the material is in and what requirements are necessary for the finished weld.
In many cases, there are fewer, less complicated considerations for the application than initially thought. As always, when questions arise about the best approach to a welding application, consult with a trusted filler metal manufacturer or welding distributor for assistance.