QUANTITATIVE PHASE ANALYSIS OF ARMOUR STEEL WELDED JOINT BY X-RAY DIFFRACTION

Ultra-high tensile strength is characteristic of armour steel and in order to preserve this strength its welding process is of paramount importance. Austenitic filler material is traditionally used for welding of armour steel, yet it has lower mechanical properties than the base material, i.e. the filler material is the weakest point of the welded joint. Moreover, due to the plastic deformation at the crack tip austenitic filler material gets transformed into martensite during fatigue crack propagation. An amount of austenite transformed into martensite is directly related to crack growth resistance in the weld metal. In order to quantify martensite phase formed during the crack propagation under the effect of fatigue load, we employed method of X-ray diffraction. Diffractograms were recorded in Brag–Brentano θ :2 θ reflection geometry on a Philips PW 1820/30 X-ray diffractometer employing monochromatic CuK α radiation (30 kV, 30 mA) in the range 40°-60° 2 θ . For the quantitative phase analysis RIR method was subsequently employed. From the obtained data, martensite to austenite ratio was calculated for the fracture surface. Thereafter the 0.05 mm thick layer was removed from specimen surface and the diffraction patern was recorded again. This procedure was repeated till 25% of the martensite remained in two-phase mixture. α´ martensite was detected at distances up to 0.25 mm under the fracture surface. The greatest transformation of austenite into α´ martensite was 55%, seen on the fracture surface. The amount of α´ martensite declines with a distance by an average of ≈5%/0.05 mm, in the depth perception tests. At the distance of 0.25 mm, the amount of transformed austenite fell to 24%.