Weldability and embrittlement of steels

Weldability of steels

Weldability is the ease with which a steel can be welded to give a sound joint of adequate strength and toughness. Poor weldability arises from the hard, brittle martensite that forms in the heat-affected zone (HAZ) on rapid cooling.

Factors affecting weldability:

• Carbon content: below 0.25% C is readily weldable; above 0.45% needs preheating and post-weld heat treatment.
• Alloying elements: Mn, Cr, Mo, Ni raise hardenability, increasing the risk of cracking. This is expressed by the carbon equivalent:

$$CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}$$

Steels with CE below about 0.4 are generally weldable without preheat.

• Thickness and cooling rate: thick sections cool faster.
• Hydrogen, sulphur and phosphorus: promote cracking and hot shortness.
• Remedies: preheating, low-hydrogen electrodes, controlled heat input, post-weld heat treatment.

Embrittlement

• Temper embrittlement: alloy steels (containing Ni, Cr, Mn) held or slowly cooled between about 375 and 575 °C lose toughness, because impurities (P, Sb, Sn, As) segregate to prior-austenite grain boundaries, causing intergranular fracture. Remedies: add Mo (about 0.5%), keep impurities low, quench rapidly through the range.
• Hydrogen embrittlement: atomic hydrogen absorbed during pickling, plating or welding diffuses into the steel and reduces ductility and causes delayed cracking at stress well below yield. High-strength steels are most susceptible. Remedy: baking (about 200 °C), low-hydrogen processes.