The effect of low-temperature tempering and surface condition on hydrogen susceptibility of direct-quenched 500 HBW steel

A direct-quenched high-strength steel with an auto-tempered lath-martensitic microstructure and nominal hardness level of 500 HBW is investigated with a novel in-situ hydrogen-charging tuning-fork testing method. Tests are conducted with notched tuning-fork specimens, which are stressed with a loadcell clamping system and in-situ charged with hydrogen until failure (3% NaCl + 0.3 g/l NH₄SCN, -1.2 V). During charging, a crack initiates at the bottom of the notch (stress concentration factor, k_tn ≈ 3.6) and eventually propagates through the specimen. Crack initiation time (t_i), time-to-fracture (t_f), and crack propagation rate are determined from the time-force data¹.

The effect of specimen surface condition on hydrogen susceptibility is studied with differently manufactured notches. The first notch type is manufactured by wire cutting, and the second is machined by milling. Both types of specimens are studied in the as-delivered state (DQ) and low-temperature tempered state (DQ+T). Tempering is conducted at 200 °C (LTT) with both taking the samples out of the furnace as soon as the LTT temperature is reached and after 2 h holding at LTT.

Without tempering (DQ), both notch types show approximately the same t_f, but t_i is significantly lower with the milled notches. After tempering treatment, hydrogen resistance of the specimens with wire-cut notches improves drastically, but the performance of the specimens with milled notches is unaffected. With longer cracking times, fracture surfaces show a change in the crack propagation mechanism from brittle transgranular quasi-cleavage to pronounced ductile fracture. The carbide structures and residual austenite contents of DQ and DQ-T steels are further investigated with TEM and HE-XRD.

Because of the differences in the results with the two notch types, separate clamping tests were conducted without hydrogen charging. Specimens were clamped for a 1 min period, relaxed, and the cross-cut notch region was subsequently studied with FESEM and EBSD analysis. Investigation shows that milled notches have initial microcracks after clamping. Pre-existing microcracks explain the shorter t_i times of the untempered specimens and unchanged results after tempering. Microhardness measurements of DQ specimens show that the sub-surface region of the milled notches has higher hardness in comparison to the wire-cut specimens, which have a softer surface layer at the bottom of the notch. The formation of initial cracks in the milled notches during clamping is the result of plastic deformation (cold work) that is caused by milling, while the heat input in wire cutting causes a softer heat-affected zone at the bottom of the notch.