Consumables Corner: Solving weld crack issues with 4150 chrome-moly steel

A: This problem is not at all uncommon when welding on chromium-molybdenum (CrMo) or high-strength, low-alloy steels. First you must understand the material properties.

The AISI/SAE 1018 steel fits into the low-carbon (0.05 to 0.3 percent carbon) grade and has a carbon range of 0.14 to 0.2 percent. It has a minimum yield strength (YS) of 53,700 PSI and an ultimate tensile strength (UTS) of 63,800 PSI. Since the carbon content is relatively low, and there are no other significant alloying elements that could drastically increase the carbon equivalency, there isn't much else to consider during welding. Although, due to the thickness of your application, we do recommend some weld preheat.

AISI/SAE 4150 is considered a 41XX series CrMo alloyed steel. Various grades have slightly different alloying compositions and are identified by the last two numbers of the SAE designator, which represents the carbon content. It is also considered a medium-carbon (0.3 to 0.8 percent carbon) steel grade with 0.48 to 0.53 percent carbon, hence the “50” in the name. The mechanical properties are greater compared to the 1018 steel. The 4150 steel has a minimum YS of 55,100 PSI and a UTS of 106,000 PSI in the annealed condition. Because of the elevated carbon content with the addition of alloys and a calculated carbon equivalency of approximately 0.88, preheating before any welding takes place is required along with considerations after welding.

Regarding your application, three things make it more challenging. First, the considerable material thickness equates to fast cooling rates. Second, a high-strength alloyed steel typically has a microstructure that is susceptible to martensite formation and potentially hydrogen-induced cracking. Third, a pipe-to-plate weld structure is considered to be a high-restrained weld joint configuration.

You stated that you were previously using an ER80 product that contained 1 percent nickel and never experienced cracking. Depending on weld parameters, heat input, and weld size, the nickel content may be responsible for the resistance to postweld cracking. Changing to the ER70S-6 electrode shouldn't have caused any issues since the 1018 plate is the governing material for filler metal matching considerations. Remember, the general rule is to match to the lower-strength material.

A few variables are missing that could help us analyze the issue, such as your base material condition (i.e., annealed or normalized), shielding gas type, welding electrode type or process, weld bead size, technique, and all related weld parameters. Additionally, what postweld procedure did you follow?

With the high carbon equivalency of the 4150 material and the 2 in. thickness, we recommend you apply a minimum preheat of 600 degrees F before any welding activities, including tack welding.Oftentimes people skip preheat before tack welding and fit-up. Tack welding is still considered welding, and with the extreme temperature of the welding arc and cool base material, the tack weld cools incredibly fast along with the heat-affected zone (HAZ), resulting in small cracks in the susceptible material. Once a crack has formed, there is nothing to stop it from propagating and causing a failure.

Before fit-up, apply a uniform and thorough soaking preheat. Tack weld the components into place while maintaining the minimum preheat. Weld the joint completely before cooling is allowed, using weld beads equivalent to a ¼- to 5/16-in. fillet. Make sure the welds are continuous around the pipe if possible, or weld opposing sections to balance the weld shrinkage stresses.

For example, if you weld a section from the 12 o’clock to the 3 o'clock positions, you should weld the next section from the 6 o’clock to the 9 o'clock positions, and so on. After full-weld, maintain the minimum preheat temperature for one hour per inch of thickness (two hours in your case) to allow any potential hydrogen to diffuse out of the material. Afterwards, remove the preheat source and thoroughly cover the weld with proper insulating materials, such as several layers of fire blankets or ceramic insulation.