How Heat Treatment Can Affect Welded Joints

07 October 2019

Much though engineers wish the opposite were true, heat treatment work can weaken welded joints. Since welding equipment uses thermal energy to fuse parts together and add filer metals, the application of more heat can further alter a weld’s jointing properties. A frustrating fact indeed, especially since heat treatment processing uses an inordinate amount of facility resources.

Furnace Heat Can Introduce Weld Cohesion Problems

It’s not as if a project leader can skip a mandatory heat treatment operation, not even if the parts involved have been welded. At the same time, a furnace foreman can’t indiscriminately send a number of welded components forth, not without initiating a handful of process-essential countermeasures. Without these measures, a perfectly placed weld seam could age and weaken. The alloy transformative energy could even trigger a corrosive effect, which eats into a weld’s face. To flush out such joint undermining concerns, quality assurance techs often use optical microscopy instruments to check out sample welds.

Putting Weld Grain Under The Microscope

Let’s say this is an equipment manufacturing project. The heat treatment phase of the job can only be done after the parts are welded. The arc welding gear, whether it uses TIG or MIG technology or some other process variant, creates a perfect butt joint with an equally perfect bevelled edge. The assembled parts are strong and tough. After the heat treatment, the jointed material pieces become even more corrosion resistant and harder. That’s all well and good, but what about that seam? After having been in the furnace, is it still as tough as ever? Or is it now a fatigue-weakened joint? Well, back under the optical scope, the carbon content and parent metals in a weld create a complex mix of ferrite-carbide solids. In view of this, welded parts can become every bit as tricky to heat treat as solid metal workpieces.

If a welded metal frame is properly heat treated, it retains jointed strength and seam hardness. However, if an improperly applied heating or cooling technique is applied, the joint could take on process stresses or material fatigue. With stress issues, crack propagation problems crop up, and with material fatigue problems, perhaps because the carbon content has altered or the ferrite platelets have transformed, the weld face and root weaken. Depending on the quenching and tempering method used, the filler metal in a weld could either soften or become brittle. Neither arc-fused bonding state is desirable. Of some interest, PWHT (Post Weld Heat Treatment) techniques can be used to equalize a filler metal’s complex ferrite-carbide blend. Used judiciously, weld stress and hydrogen diffusion issues can be greatly minimized.

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