Sep 22, 2016

Tips for Successfully Welding 4130 Heat-Treatable Low-Alloy Steel

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 treating. The material’s varying mechanical properties, however, can make it challenging to select a filler metal that matches its strength in a given condition. For that reason, there are many different ways to weld 4130, 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.

Lead Photo
Because of its high hardenability, 4130 steel responds well to heat treating, which offers advantages for many applications. But, this material can also vary in mechanical properties as a result, making it challenging to select a filler metal that matches its strength in a given condition.

Common ways to weld 4130 steel
4130 steel 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 strength for annealed 4130 steel is 90 ksi, while the material in a normalized condition is around 100 ksi; quenched and tempered 4130 steel is stronger, offering 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 steel and the demands of the application. 

Strength undermatch: When a part design and/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 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. 

Common Mild Steel Filler Metals for Strength Undermatch Figure
Figure 1. Recommendations per American Welding Society (AWS) classification. 

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 to 90 ksi tensile strength would be 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.

Filler Metals for Matching Annealed Strength Figure
Figure 2. Recommendations per AWS classification. 

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, there are high carbon levels in 4130 materials (which is what makes them heat treatable); as a result, they are more sensitive to cracking. Therefore, if the application does not require post-weld heat treatment, there is not an advantage to using a chemistry match with 4130 filler metals

There are other filler metal options that can be used successfully 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 metals 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. 

Additional tips for welding 4130
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.

Preheat and Interpass Temperature Figure
Figure 3. Preheat recommendations. 

Slow cooling of welded parts can be controlled with blankets, ovens or other methods. This practice offers the same advantages of preheating — to reduce the chance of unwanted microstructures forming, which helps the piece resist hardening or cracking.

Post-weld stress relief is another method to help prevent cracking and defects in the finished weld. Thin material (less than 1/8 inch in thickness) typically does not require stress relief because cracking is less of a concern. Thicker materials are commonly stress-relieved at 1,100 to 1,250 degrees Fahrenheit 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 the welding application, consult with a trusted filler metal manufacturer or welding distributor for assistance.