There are two methods by which both high tensile strength and toughness is achieved - by micro-alloying, adding small amounts of strong carbide and nitride formers and by very careful control of the rolling temperature - controlled rolling or thermo-mechanically controlled processing (TMCP steels).
The highest strengths are achieved by a combination of the two methods. The aim of both methods is to produce as small a grain size as possible, fine grain giving the best notch toughness and each halving of the grain diameter producing a 50% increase in tensile strength.
Improved weldability is an additional objective and this is achieved by reducing the hardenability of the steel, the carbon content of some steels being lower than 0.05%C, and reducing undesirable elements such as sulphur and phosphorous to as low a level as possible.
To compensate for the loss of carbon and to increase tensile strength small additions of alloying elements such as niobium (<0.10%), titanium (<0.030%) and vanadium (<0.15%) are made, perhaps also with small amounts of molybdenum, chromium, copper and nitrogen. These elements are strong carbide and nitride formers, producing a fine dispersion of stable precipitates that inhibit grain growth during hot rolling and assist in nucleating fine grained ferrite during cooling.
These elements also provide some increase in strength by precipitation hardening. Controlled rolling by the TMCP hot rolling method may also be used to provide additional grain refinement and hence an increase in tensile strength and toughness. TMCP is carried out at a temperature about or just below the recrystallisation temperature of the steel i.e. below about 900°C, resulting in elongated crystals of austenite. Accelerated cooling from the rolling temperature then causes very fine grained ferrite to form on the austenite grain boundaries.
Despite the improved weldability of these steels there are some fabrication problems. Firstly, hydrogen induced cold cracking.
The low carbon content - and hence low carbon equivalent, sometimes less than 0.30CEv - means that these steels have a low sensitivity to hydrogen cold cracking (see Job Knowledge 45 but note that the standard IIW carbon equivalent formula is not valid for all of these steels and cannot always be relied upon when calculating preheat temperatures).
The HSLA steels can therefore be welded with lower preheats than would be permitted for conventional carbon-manganese steels, despite their higher strength. The highest risk of cold cracking in these types of steels is therefore in the weld metal, rather than the HAZ. There are several reasons for this; a) The high strength of the parent metal means higher residual stresses during welding, b) To match the tensile strength and toughness of the parent steel, the filler metals need to be more highly alloyed and therefore will have a higher CEv, perhaps as high as 0.6CEv (IIW) if matching the tensile strength of a 700MPa yield steel with an E11018-G electrode. c) The weld metal transforms from austenite to ferrite at a lower temperature than the parent steel (it is generally the other way round in a conventional carbon-manganese steel) meaning that any hydrogen in the HAZ is rejected into the still austenitic weld metal which has a high solubility for hydrogen. A preheat based on the weld metal composition is therefore advisable and low hydrogen techniques must be used. The exceptions to this rule are those HSLA pipeline steels specifically designed to be welded with cellulosic electrodes. Advice regarding the preheat temperature for specific steels should be sought from the steel manufacturer.
Secondly, even though steels generally have very low levels of sulphur, the steels containing less than 0.05%C may suffer from solidification cracking in the root pass of butt joints, particularly if the root bead is deposited at a high welding speed. The reason for this is that high dilution of the filler metal produces a weld metal low in carbon. This low carbon content in its turn leads to excessive grain growth of the austenite during welding and these large grains increase the risk of centre line solidification cracking in the root bead. This problem appears to be most prevalent in pipe butt joints welded using cellulosic electrodes, probably due to it being possible to use a fast, vertical-down welding technique.
Thirdly, toughness and strength in the HAZ can be an issue. The steel manufacturer takes great care to control rolling temperatures and cooling rates to provide the desired properties. The component is then welded, producing a heat affected zone that has experienced an uncontrolled cycle of heat treatment. The microstructure in the HAZ will vary with respect to the composition of the steel and the welding process heat input. A high heat input will promote grain growth and this will have an adverse effect on both strength and toughness. As a rule of thumb, heat input should be restricted to around 2.5kJ/mm maximum and the interpass temperature maintained at 250°C maximum, although some of the steels containing titanium and boron can tolerate heat inputs as high as 4.5kJ/mm without undue loss of strength. For a definitive statement on heat input control the advice of the steel manufacturer should be sought.
These steels must under no circumstances be normalised or tempered although post weld heat treatment (PWHT) is often a requirement when the component thickness is greater than some 35 to 40mm. Care needs to be taken if PWHT is applied that the soak temperature does not exceed 600°C; a temperature range of 550°C to 600°C is often specified. The reason for this is that many of the TMCP steels are accelerated cooled to a temperature of around 620°C; heat treating at or close to this temperature will result in a substantial reduction in tensile strength due to over-tempering. The same restriction applies to any hot working activity - plate must not be hot rolled and the temperature of local heating for correction of distortion must not be allowed to exceed 600°C.
Further advice on the welding of these steels can be found in the trade literature and in the specification EN 1011 Part 2 Welding - Recommendations for welding of metallic materials: Arc welding of ferritic steels.
This article was written by Gene Mathers.