Frequently Asked Questions
TMCP (Thermo-Mechanically Controlled Processing) strengthens a steel of given composition by hot working and sometimes by fast cooling as well ( click herefor more information on TMCP steels). The strength and grain size may be damaged by the heat of welding. There are two main types of damage that are of concern. Firstly, softening can occur in TMCP steels on welding, as the cooling rate of welding is usually slower than that of the original processing. Secondly, as with other steels, grain coarsening in the high temperature heat affected zone (HAZ) can result in reduced toughness.
Although softening is often observed in the outer regions of TMCP steel HAZ's, a reduction in joint strength is not seen with all TMCP steels(1-3). The most susceptible are those that use accelerated cooling as part of the TMCP route. There is a heat input restriction applied to those steels that will experience joint strength reduction upon welding; typically a maximum heat input of 2.5kJ/mm for 15mm thickness. This value is dependent on the steel grade, and is more critical for higher strength steels that have had more rapid cooling.
Generally, TMCP steels are good at resisting a reduction in toughness as a result of welding, because such steels usually have a low carbon content. Some TMCP steels are excellent at resisting the toughness decrease, even at high heat inputs, as they have been processed to contain particles which control grain growth (See FAQ: Why is the Ti:N ratio specified in steel specifications?). These steels also have sufficiently high carbon content, or carbon equivalent levels to ensure that joint strength is not impaired at high heat inputs. Such steels are designated 'high heat input resistant steels'.
References
- Shiga C:'Trends in steels for shipbuilding - the growing use of advanced steel', IIW Shipbuilding seminar, Odense Denmark, 17-19 April 1996.
- Yajima H, Tada M, Kajimoto K, Kawamura A, Noda S and Nawata T: 'Extensive application of TMCP-manufactured high tensile steel plates to ship hulls and offshore structures' Mitsubishi Heavy Industries Ltd Technical Review Vol 24, No 1, Feb 1987.
- Yurioka N: 'TMCP steels and their welding' Welding in the World, Vol 35, No 6 pp.375-390, 1995
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