Effects of heat input on the mechanical properties of butt-welded high and ultra-high strength steels

This paper presents an investigation into the mechanical properties of ultra-high strength steels (UHSSs) (with nominal yield stress of 960 and 1100 MPa) after welding. Seven weld thermal cycles were simulated using a Gleeble 3800 thermal-mechanical machine. These cycles represented the temperature-time history of the joint at various distances from the weld fusion line (FL) in a typical gas metal arc welding (GMAW) process. The mechanical properties such as Vickers surface hardness, uniaxial tensile behavior and Charpy impact toughness were examined. Microstructural evaluation using field emission scanning electron microscopy (FESEM) was also conducted. According to the results, the S960 which was a direct-quenched type of steel, showed a considerable reduction in its hardness and tensile strength up to 29 and 32%, respectively. On the other hand, the S1100 which was manufactured via quenched and tempered process, showed only a minor degree of softening at far distances from the weld FL (up to 4%) followed by 2% tensile strength reduction, and hardening close to the FL (up to 13%). Microstructure analysis has been carried out to provide insight to the change of mechanical properties in UHSS after welding.

Mechanical properties and microstructural evaluation of the heat-affected zone in ultra-high strength steels

Effects of pre-fatigue damage on mechanical properties of Q690 high-strength steel

This paper focused on the effect of different laser scanning speed (4 mm/s, 5 mm/s, 6 mm/s and 7 mm/s) on the microstructural evolution of direct laser deposition (DLD) 12CrNi2 alloy steel, and analyzed the relationship between microstructure and performance of DLD-processed samples. The results showed that the microstructure in the middle of as-deposited samples consisted of a large amount of bainite, a small amount of martensite (M) and ferrite (F). With the increase of laser scanning speed, the fraction of ferrite decreased from 55.6% to 14.7%, while that of martensite increased from nearly 0% to 4.9%. Besides, as increasing the laser scanning speed, granular bainite (GB) transformed into lath bainite (LB) due to the increase of cooling rate, and the fraction of LB reached the maximum of 29.9% when the scanning speed was 7 mm/s. In addition, the functions about the relationship between laser scanning speed and phase fractions were fitted in order to provide a theoretical basis for the design of DLD process parameters. EBSD maps of as-deposited samples exhibited anisotropy due to the complex heat flux direction during the multi-layer laser deposition process. With the increase of laser scanning speed, the grain size showed a downward trend from 5.89 µm2 to 3.44 µm2. The sample fabricated at 7 mm/s contained more LB and M, leading to the highest mean microhardness of 355 ± 6 HV0.2. The sample fabricated at 6 mm/s exhibited the best wear resistance due to its optimum combination of hardness and toughness. Because of a large amount of ferrite with optimal toughness, the sample fabricated at 4 mm/s had the best impact toughness of aku = 80 J/cm2.

The effect of laser scanning speed on microstructural evolution during direct laser deposition 12CrNi2 alloy steel

Direct manufacturing techniques, such as directed energy deposition (DED), are able to produce complex components efficiently. In this study, microstructure evolution and impact toughness of DED 300M ultra-high strength steel are investigated. The results show that the microstructure of the as-deposited DED 300M ultra-high strength steel is mainly composed of martensite and some blocky bainite. The micro-segregation of elements is observed within the interdendritic area. After heat treatment, the microstructure becomes uniform and consists of martensite and lower bainite. The impact toughness of the as-deposited DED 300M ultra-high strength steel is 9 J/cm2, while it is significantly increased to 25 J/cm2 after heat treatment. Furthermore, it is observed that the fracture mode of the as-deposited sample is quasi-cleavage fracture. During the process of propagation, the main cracks would go across the martensite packet and deflect in the another one, and secondary cracks also deflected in the high-angle grain boundaries. By contrast, the fracture mode of heat-treated DED 300 M steel is ductile fracture.

Effect of microstructure on the Charpy impact properties of directed energy deposition 300M steel

Impact toughness properties of four kinds of high strength steel (HSS) were investigated with the nominal yield strength 460, 690, 800 and 960 MPa, respectively. The specialty of impact toughness among base metal, heat affected zone (HAZ) and weld metal were also compared and discussed. The correlations between the impact toughness of HSS and their nominal yield strength, plate thicknesses, testing temperature, welding methods were further studied. Combined with the scanning electron microscope (SEM) fracture surface observation, the microstructural features and corresponding fracture mechanisms have been analyzed. The results show that the impact toughness for HSS base metal deteriorated with the increasing nominal yield strength. The impact toughness of weld metal for HSS is much lower than that of the corresponding base metal. The impact performance of HAZ is discretized due to the welding thermal cycling on the HAZ where significant changes in grain structure and properties occur. The microcosmic fracture surface of HSS is examined to explain the reduction in impact performance under low temperature.

Low temperature impact toughness of high strength structural steel

Experimental research on fatigue performance of high-strength structural steel series

This paper presents an experimental investigation into mechanical properties and fatigue performance of butt-welded high strength steel plates. Four typical high-strength steels (HSSs) were selected, with steel grades of 460, 550, 690, and 960 MPa. Mechanical properties were measured through monotonic tensile tests, cold bending tests, Charpy V-notch impact tests and hardness tests. A total of 114 fatigue tests were conducted on butt-welded high-strength steel plates. Comparisons of mechanical properties and fatigue performance are made among HSSs tested and conventional strength steels (CSSs) with a yield strength around 235 to 355 MPa. The natural slope of S–N curves for HSS butt-welded connections varied from 3 to 5. Two fatigue design approaches are recommended with one based on natural slope and one based on a fixed slope of 3 as for CSSs.

Experimental investigation on fatigue behavior of butt-welded high-strength steel plates

This study presents an experimental investigation of the fatigue performance of non-load-carrying transverse fillet-welded attachments made of high-strength steel (HSS). Four typical HSSs were selected, with steel yield strength of 460 MPa, 550 MPa, 690 MPa, and 960 MPa. A total of 88 high-cycle fatigue tests were conducted. Based on the analysis of the morphological details of the fracture surfaces and the failure modes, three different methods for fatigue assessment, such as the nominal stress method, the effective notch stress method and the strain energy density method, were applied to directly compare the fatigue performance of HSS and conventional strength steel (CSS, with a yield strength of approximately 235–355 MPa). HSS transverse fillet-welded attachments have demonstrated excellent fatigue resistance compared with specifications (FAT 80) in the design codes by using the nominal stress method, the natural slope of the S – N curves is nearly 4.0, rather than 3.0 prescribed in the current design codes for fatigue assessment of welded structures. For the notch stress method, HSS transverse fillet-welded attachments show a greater increase in fatigue life for low notch stress ranges (approximately Δ σ k ≤ 400 MPa). However, for high notch stress ranges (approximately Δ σ k > 400 MPa), the increase in fatigue life is limited. The results with the strain energy density method are similar to those with the notch stress method. • Fatigue tests of transverse fillet-welded attachments made of high-strength structural steel (HSS) series were carried out. • Fracture surface features and typical failure modes were compared to reveal the crack-propagation behaviors. • Three different methods for fatigue assessment were applied to compare the fatigue performance of various HSS. • Key factors affecting the fatigue properties of HSS transverse attachments were discussed in detail. 

Fatigue performance of transverse welded attachment of high-strength structural steel series

High-strength steel (HSS) has been increasingly applied in structural engineering, while the work on residual stress of HSS sections is still insufficient. Limited research is available for welded H-sections above 690 MPa while with a plate thickness of more than 10 mm. This paper deals with the residual stress distributions of welded H-sections with flame-cut edges, which are fabricated from HSS plates with yield strength of 690 MPa and 960 MPa and thickness of 10 mm–20 mm, using both experimental and numerical method. The experiments showed that the welded tensile residual stresses in the weld region are not significantly affected by sectional dimensions and steel grade. Verified by comparing with the experimental results, three-dimensional (3D) finite element (FE) models using sequentially coupled thermal-stress analysis was adopted for parametric study of compressive residual stress of welded H-sections made of 690 MPa HSS. The results showed that the compressive tensile stress is negatively correlated with plate thickness as well as width-thickness ratio. Through regression analysis, formulae were proposed for estimation of compressive residual stress in the flange and web, respectively. Finally, residual stress distribution model was developed, which was proved effective by comparing with the experimental and FE results.

Lichao Niu

Papers

Low temperature impact toughness of high strength structural steel

Experimental research on fatigue performance of high-strength structural steel series

Experimental investigation on fatigue behavior of butt-welded high-strength steel plates

Fatigue performance of transverse welded attachment of high-strength structural steel series

Experimental and numerical study on residual stress distributions in welded H-sections of High-strength steel