Characteristics of the heat-affected zone in ultra-fine grained steel during ultra-narrow gap GMA welding. Softening zone and microstructures of the heat-affected zone in ultra-fine grained steel

摘要

This paper describes an investigation of the relationship between the peak temperature and microstructural changes as well as the HAZ softening behaviour due to ultra-fine grained ferrite coarsening under the effect of the welding thermal cycle of ultra-fine grained high-strength steel featuring high strength enhancement to 800 MPa level through ferrite grain size refinement to around 0.9 im in a basic (Fe-C-Si-Mn) SM490 steel chemistry. The peak temperature of the HAZ in UFG steel during welding by the ultra-narrow gap gas metal arc welding process (during a cooling time of around 4.5 sec from 1073 K to 773 K in the weld bond) developed to allow welding to proceed at high efficiency and low heat input specifically to reduce the HAZ softening width is measured and analysed, and the relationship between the softening behaviour, ferrite grain size changes, and microstructural characteristics is clarified. The results are compared with those obtained for SM490 steel having a similar composition and ferrite-pearlite microstructure. To examine the foregoing results in more detail, synthetic HAZ simulations are also run to establish the relationship between the softening behaviour and microstructures. The results obtained may be summarised as follows: To analyse the HAZ softening characteristics of UFG steel, the peak temperature and cooling rate are measured at different HAZ positions and evaluated by use of a heat conduction model. The results show the peak temperature range causing softening to extend from a temperature of around 920 below the Ac, point to 1300 K, with heaviest softening occurring at around 1100 K. The hardness at the location of greatest softening is some 40HV0.5 lower than the base metal hardness, being the same as the hardness transition of SM490 steel in the temperature range above around 1370 K. The massive fine ferrite grain size starts to coarsen from a peak temperature of above 920 K below the Ac_1 point, having much the same size of 10 mum as SM490 steel at a temperature of around 1300 K. The results show that, for UFG steel hot-rolled at a final pass temperature of 773 K: 1) the steel retains thermal stability up to 920 K in respect of its ferrite grain size; 2) as compared with the ferrite of SM490 steel, the effect of any difference in the rolling and working history under controlled pressure on the ultra-fine grained ferrite grain size is retained up to a temperature of 1300 K. The microstructure of UFG steel composed of ultra-fine grained ferrite and uniformly dispersed ultra-fine grained cementite has a two-phase structure consisting of ferrite and cementite at a peak temperature below the Ac_1 point, a three-phase structure consisting of ferrite, cementite, and M-A in the range from Ac_1 to Ac_3, and a two-phase structure consisting of ferrite and finely dispersed M-A from just above Ac_3 to 1250 K. At a temperature above 1250 K, the steel has three phases involving bainite being included in massive ferrite and M-A. At a temperature above 1370 K, no massive ferrite is observed, the microstructure being composed of low-carbon martensite, bainite, and M-A including a small amount of bainitic ferrite. On the other hand, SM490 steel having a ferrite grain size of around 10 im and striated pearlite distributions shows some pearlite being transformed into M-A at a temperature above Ac_1 to obtain the three phases of ferrite, pearlite, and M-A. At 1020 K, the pearlite is fully transformed into M-A. Above Ac_3, the steel has a mixed microstructure consisting of massive ferrite refined to around 7 mum, bainite, low-carbon martensite, and M-A. At a peak temperature above around 1370 K, SM490 steel has much the same microstructure as UFG steel. Within the peak temperature range from above 920 K below Ac_1 to below Ac_3, UFG steel shows heavy scatter in its calculated ferrite grain size. This is attributable to the formation of a mixed microstructure consisting of ultra-fine grain A showing some slight growth