TWI Frequently asked questions
by Joanna Nicholas
For a given strength level, a TMCP steel will have a lower alloy content than a normalised steel, and thus will be more readily weldable with regard to avoidance of HAZ hydrogen cracking, and achievement of maximum hardness limits.
Heat affected zone fracture toughness properties are generally better in TMCP steels than normailised steels and adequate fracture toughness can often be obtained up to higher heat inputs. Indeed, some "high heat input resistant" grades of steel are made by a TMCP route.
There will be some degree of softening in the heat affected zones of TMCP steels after welding. Reduction in joint strength, however, is unusual, in those that have not had accelerated cooling as part of the manufacturing process. TMCP steels that have been manufactured using an accelerated cooling method to achieve the desired properties are not generally resistant to high heat inputs, and a degradation of properties can occur on welding. This is because the cooling rate in the weld region may be slower than that of production. In these steels it is important that the cooling rate is high, so that the grain size of the weld and HAZ can be maintained to give the desired properties. This is achieved by the use of moderately low heat input levels, typically ≤ 2.5kJ/mm for 15mm plate in these steels.
Further information
FAQ:
Is there a restriction on the heat input that can be used for the welding of TMCP steels - if so, why? FAQ: What are TMCP (Thermo-Mechanically Controlled Processing) steels?
