African Fusion August 2015

To maintain HSS weldment qualities, very strict control of welding parameters is necessary. Transformations of these metals at the end of thewelding operation, which significantly affect the microstructure and mechanical properties and fatigue life, are difficult or practically impossible to reverse despite post-weld heat treatments [7 and 8]. Because of the different methods that are used to manufacture the various high-strength steels available, welding conditions that are applicable to one steel may not be applicable to another [7, 9 and10]. This is an important factor to consider in the case of welding dissimilar metals. Figure 2 shows how two different high-strength steels from different weldability zones may ex- hibit different challenges in welding, causing them to require their own specific welding procedures. The carbon content is a factor to be considered alongwith other austenite stabilisers, as has already been noted above. Risks are particularly associated with a significant increase in the diffusion quantity of brittle component elements during very rapid cooling, especially around the fusion zone (FZ) and heat-affected zones (HAZ) [1]. These risks are the funda- mental reason for the establishment of appropriate welding procedures for thesemetals in general and particularly for the dissimilar welding of high strength steels. Welding procedures in the case of high-strength steels include several key factors. Among them, there is the use of carbon equivalent (CE) equations to evaluate weldability. Grafille’s diagram in Figure 2 is an illustration for determining the degree of difficulty of welding ahigh-strength steel. Inaddi- tion to carbonequivalent, ananalysis of the thermal conditions of thewelding processmust be added, as they define theweld- ing cooling time and conditions. The admissible temperature limit takes into account the preheating and possible inter-pass temperatures. It is possible to define an acceptable welding lobe for each HSS on the basis of the temperature of the pre- heat or inter-passes and the heat input required. Software thusmakes it possible toplot the allowableweld- ing limit frame of the HSS. Figure 3 presents the two welding frames of S355K2+N and high S1100QL grades given by weld- ing software based on the CE method [12]. In the case of a dissimilar weld between these two metals, it is necessary to take care to keep the temperaturewithin S1100QL limits, since its welding temperature condition is smaller and inside the allowable heat frame of S355K2+N. This example is evidence that high-strength steels have more restrictions on welding conditions than mild steel and require a method to ensure these conditions are respected. Tables 1, 2 and 3 give an idea of the variety of high-strength

Figure 2: Weldability of structural steels by the Graville diagram [11].

Figure 3: Welding frames of S335K2+N and S1100QL grades [12].

steels. Each table shows the ultimate tensile strength (UTS) limits, themethod used to increase the strength and examples of existing grades. It can be observed that themethods utilised inmanufacturingbecome increasingly complex as the strength increases. In addition it can be seen that strengths are becom- ing higher – up to 1 300 MPa. Welding consumables Apart from the welding category that does not use an elec- trode for welding, it should be noted that the filler metal plays a leading role in fusion welding. Filler metal is used for



(UTS<600 MPa)

Characteristic features

Example

BH (bake hardening): increase strength during paint treatment by controlled carbon (C) ageing. IF-HS (high-strength interstitial free): increase strength via manganese (Mn) and phosphorus (P) additions. IS (Isotropic): increase in strength using isotropic flow behaviour, micro-alloyed with Ti or Nb. CMn (Carbon-manganese): strengthened with an increase of C, Mn and Si addi- tions for solid solution strengthening. HSLA (high-strength low-alloy): strengthened by micro-alloying with Nb or Ti. P (re-phosphorised): phosphorus-alloyed high-strength steels.

BH 280/400 (YS: 300-360 MPa, UTS: 420-480 MPa)

IF 300/420 (YS: 320 MPa, UTS [min]: 420 MPa)

H220PD (YS [min]: 220 MPa, UTS [min]: 420) HC260I (YS: 220/260, UTS [min]: 300/380)

CMn 440 (YS [min]: 295 MPa, UTS [min]: 510)

HSLA 550/650 (YS: 585 MPa, UTS [min]: 650 MPa)

Table 1: High-strength steel (HSS) characteristics and examples.

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August 2015

AFRICAN FUSION