A B S T R A C T First, fatigue tests were performed on butt-welded joints made of novel direct quenched ultra high strength steel with high quality welds. Two different welding processes were used: MAG and Pulsed MAG. The weld profiles, misalignments and residual stresses were measured, and the material properties of the heat-affected zone were determined. Fatigue tests were carried out with constant amplitude tensile loading both for joints in as-welded condition and for joints after ultrasonic peening treatment. Finally, in fatigue strength predictions, the crack initiation phase was estimated using the procedures described by Lawrence et al. [Lawrence F V, Ho N J and Mazumdar P K (1981) Predicting the fatigue resistance of welds. Annu. Rev. Mater. Sci,11,. The propagation phase was simply estimated using S-N curves for normal quality butt welds, which may contain pre-existing cracks or crack-like defects eliminating the crack initiation stage.Keywords fatigue; local approach; ultra high strength steels; weld profile. N O M E N C L A T U R Ea * = material parameter ASW = as-welded condition A 5 = elongation, permanent extension of the gauge length after fracture, % b = fatigue strength exponent c = fatigue ductility exponent C = fatigue capacity CEV = carbon equivalent value e = axial misalignment E = Young's modulus FAT = fatigue class, fatigue strength corresponding to two million cycles GMAW = gas metal arc welding HAZ = heat-affected zone HB = Brinell hardness number [kg/mm 2 ] HV10 = Vickers hardness number obtained using a 10 kgf force [kg/mm 2 ] IIW = International Institute of Weldingnotch factor K m = stress concentration factor caused by misalignments K m,e = axial magnification factor K m,α = angular magnification factor K t = stress concentration factor Correspondence: T. Nykänen. Fatigue Fract Engng Mater Struct 36, 469-482 469 470 T. NYKÄNEN et al.K w = stress concentration factor of weld l = half distance between clamps LCF = low cycle fatigue LEFM = linear elastic fracture mechanics m = exponent of S-N curve n = static strain-hardening exponent n = cyclic strain hardening exponent NDT = non-destructive testing N f = total life N i = initiation life N p = propagation life QC = quenched and cold-formable steel R = stress ratio, the ratio of minimum to maximum applied stress in a cycle R p0.2 = 0.2% proof stress R m = ultimate tensile strength SLM = structured light method t = plate thickness t 8/5 = cooling time between 800 • C and 500 • C UP = ultrasonic peening treatment α = angular misalignment angle, survival probability β = two-sided confidence interval ε = local strain range ε e = elastic strain range ε p = plastic strain range K = stress intensity factor range S = nominal stress range σ = local stress range ε f = fatigue ductility coefficient θ = weld notch angle ρ = notch root radius ρ c = critical notch root radius σ f = fatigue strength coefficient σ m = mean stress, a combination of applied and residual stresses σ res = residual stress Sub indexes: geo = mean value of log-normal distrib...

This paper reports the effects of chemical composition on the hardness of the heat affected zone of re-austenitized and water quenched steels. Heat affected zone peak temperatures in the range 300–1350 °C were simulated using a Gleeble 3800 simulator using thermal cycles appropriate to welds with cooling times between 800 and 500 °C of 12s. The maximum softening relative to the base material occurred in the intercritical and subcritical heat affected zones at the peak temperatures of 700 or 800 °C. Usually softening was greatest when the peak temperature was 700 °C. Linear regression analysis showed that carbon, and to some extent manganese and nickel, are detrimental at the peak temperature of 700 °C, but beneficial at the peak temperature of 800 °C in respect to softening relative to the base material, whereas niobium and especially molybdenum are beneficial at both temperatures. The beneficial effects of molybdenum alloying are seen down to peak temperatures of 400 °C whereas the effect of niobium microalloying is not statistically significant at peak temperatures lower than 700 °C. The softening in the intercritical, fine-grained and coarse grained heat affected zones are discussed and the effects of the alloying elements on the hardness of the subcritical heat affected zone are compared with their known effects on martensite hardness during conventional tempering.

Physical simulation of the most critical sub-zones of the heat-affected zone is a useful tool for the evaluation of the toughness of welded joints in high-strength and ultra-high-strength steels. In two high-strength offshore steels with the yield strength of 500 MPa, the coarse grained, intercritical and intercritically reheated coarse grained zones were simulated using the cooling times from 800 to 500 °C (t8/5) 5 s and 30 s. Impact and CTOD tests as well as microstructural investigations were carried out in order to evaluate the weldability of the steels without the need for expensive welding tests. The test results showed that the intercritically reheated coarse grained zone with the longer cooling time t8/5=30 s was the most critical sub-zone in the HAZ due to the M-A constituents and coarse ferritic-bainitic microstructure. In 6 mm thick ultra-high-strength steel Optim 960 QC, the coarse grained and intercritically reheated coarse grained zones were simulated using the cooling times t8/5 of 5, 10, 15 and 20s and the intercritical zone using the cooling times t8/5 of 5 and 10 s in order to select the suitable heat input for welding. The impact test results from the simulated zones fulfilled the impact energy requirement of 14 J (5x10 mm specimen) at -40 °C for the cooling times, t8/5, from 5 to 15 s, which correspond to the heat input range 0.4-0.7 kJ/mm (for a 6 mm thickness).

No abstract

No abstract