L.W. Tsay

Fatigue crack growth behavior of laser-processed 304 stainless steel in air and gaseous hydrogen

Abstract Fatigue crack growth test was performed to evaluate fatigue behavior of 304 stainless steel specimens with or without laser processing (welding and surface treatment) in air and gaseous hydrogen. As the crack propagation normal to the laser welding or scan direction, the laser-processed specimens exhibited a higher resistance to crack growth in the low stress intensity factor range (Δ K ) than the as-received steel plates regardless of testing environments. However, the marked retardation of crack growth behavior vanished for welded specimens subjected to a 850 °C/h stress relief treatment or with a shorter distance from notch tip to the weld centerline in the test. Fatigue-fractured appearance of the steel plate tested in air was composed of mainly transgranular fatigue fracture and some flat facets, along with a small amount of intergranular fracture. While quasi-cleavage fracture and few twin boundary separations were observed for the same specimen in hydrogen. On the other hand, the lower crack growth rate of laser-processed specimens in both air and hydrogen was accompanied with rubbed areas on the fracture surfaces. It was found that the extent of quasi-cleavage fracture was related to the formation of strain-induced martensite, which would contribute to an increased fatigue crack growth rate of all specimens in gaseous hydrogen.

The reliability study of selected Sn–Zn based lead-free solders on Au/Ni–P/Cu substrate

Abstract Since both Ag and In are important melting point depressants in Sn–Zn based solders, a series Sn–Zn based solders with various amounts of Ag and In additions was studied in the experiment. The melting behavior of solder alloys, wetting characteristics, coefficients of thermal expansion, microstructural evolution and long-term reliability of the selected Sn–Zn based solder on Au/Ni–P metallized copper substrate were examined. Based on the experimental result, there is little change in the melting range of Sn–Zn based solder alloys by minor addition of Ag. On the contrary, the melting point of Sn–Zn based alloys can be effectively decreased by In additions. However, the difference between solidus and liquidus temperature is broadened as the increment of In into Sn–Zn based solders. 76Sn–9Zn–15In has the lowest liquidus temperature among all alloys, and it can effectively bond the Au/Ni–P metallized copper substrate. The microstructure of 76Sn–9Zn–15In alloy soldered at 200 °C for 20 min is primarily comprised of Sn–In γ phase and needle-like ZnO 2 . Since there is no flux usage during soldering, zinc oxide cannot be avoided even the process performed under 2×10 −2 mbar vacuum environment. It is also noted that there is no interfacial reaction layer between 76Sn–9Zn–15In and Au/Ni–P metallized copper substrate after soldering. However, there is a reaction layer between 76Sn–9Zn–15In and substrate as the soldered specimen aged at 90 °C for 168 h. Its chemical composition is close to Zn-rich γ phase (NiZn 3 ) alloyed with minor Sn, In, Cu and P. For the specimen further aged at 90 °C for 336 h, there are cracks along the interface between solder alloy and electroless Ni–P layer. The oxidation of the interfacial Zn-rich γ phase plays an important role in deterioration of the bonding between 76Sn–9Zn–15In and Au/Ni–P metallized copper substrate.

A study of Sn-Bi-Ag-(In) lead-free solders

Sn-Bi-Ag-(In) solder alloys have been extensively studied in the study. The experimental results reveals that the liquidus temperatures of Sn-(1–5) Bi-(2–3.5)Ag-(0–10)In solders are between 201.7 and 225.3°C, which were higher than that of the most popular eutectic Pb-Sn solder (183°C). Additions of (5–10) wt% In into Sn-Bi-Ag solders can effectively decrease the melting point of the solder alloy. However, the gap between Ts and TL temperatures increases with the additions of Bi and In into Sn-Bi-Ag-(In) solders. Although there is no flux applied during soldering, most Sn-Bi-Ag-(In) solder alloys can well bond the Au/Ni metallized copper substrate. 94Sn-3Bi-3Ag solder demonstrates the lowest wetting angle of 45° among all test samples. Thermal expansion coefficients of both 94Sn-3Bi-3Ag and 90Sn-2Bi-3Ag-5In solders are slightly less than that of 63Sn-37Pb. Both 90Sn-2Bi-3Ag-5In/substrate and 94Sn-3Bi-3Ag/substrate interfaces demonstrate similar reaction kinetics in the experiment. The stability of the interface is greatly impaired during 90°C aging. Some locations of the electroless Ni layer break down, and new phases are formed nearby the interface during aging treatment. Initially, the growth of Ni-rich (Ni,Cu)3Sn4 phase dominates the interfacial reaction. However, the growth of Cu-rich (Cu,Ni)6Sn5 phase will dominate the reaction layer for specimens aged at 90°C for long time periods.

Influence of gaseous hydrogen on the notched tensile strength of D6ac steel

Abstract Slow displacement-rate tensile tests were performed on D6ac tempered specimens to investigate the influence of gaseous hydrogen pressure on the notched tensile strength (NTS) and the associated fracture characteristics. The susceptibility to hydrogen embrittlement of specimens at a given pressure was determined by the NTS loss, which tended to decrease as the tempering temperature increased. The NTS loss for T-300 (300°C tempered) and T-450 (450°C tempered) specimens increased with rising hydrogen pressure up to 5×105 Pa and then remained nearly the same up to the pressure of 2×106 Pa, while no deteriorated NTS in hydrogen has been found for T-600 (600°C tempered) specimens. The extent of intergranular fracture and/or the region of flat fracture on tensile fractured surfaces were consistent with the correlation of hydrogen pressure and NTS loss. The smaller the intergranular and/or flat fracture regions, the greater the resistance to hydrogen embrittlement of the specimen would be expected. Besides the effect of notch tip blunting, the excellent performance of T-600 specimens in hydrogen could be attributed partly to the presence of less continuous carbides at prior austenite grain boundaries. In contrast, T-300 specimens with grain boundary carbides in a more continuous manner were highly susceptible to hydrogen embrittlement.

Embrittlement of laser surface-annealed 17-4 PH stainless steel

Abstract Stress corrosion cracking and fatigue crack growth behavior were determined in 17-4 PH stainless steel under various metallurgical conditions, including the H900 (482°C/1 h), H1025 (552°C/4 h) aging and laser surface annealing treatments. Peak-aged (H900) specimens locally irradiated by laser beam consisted of a portion of composite region (CR), in which comprised of soft laser-annealed (LA) zones on the outer surfaces and the hard base metal in between. Slow extension rate tensile tests were performed at room temperature in a saturated H 2 S solution to evaluate the hydrogen embrittlement susceptibility of various specimens. H900 specimens show an obvious improvement in impact toughness after irradiating by laser. Regardless of testing environments, H900 specimens exhibited better tensile properties than the other specimens. Experimental results also indicated that H900 specimens had the highest fatigue crack growth rates among the specimens, particularly at low stress intensity factor range. The retardation of crack growth in the region ahead of the CR in the LA specimens was rather pronounced. For compact tension specimens tested in gaseous hydrogen, enhanced crack growth was correlated with quasi-cleavage fracture in contrast to transgranular fatigue fracture in air.

Notched tensile testing of T-200 maraging steel and its laser welds in hydrogen

Notched tensile tests were carried out to investigate the influence of gaseous hydrogen on the susceptibility to hydrogen embrittlement (HE) of aged T-200 maraging steel. The results indicated that the under-aged steel plates and their laser welds suffered great notched tensile strength (NTS) losses in hydrogen, whereas the over-aged specimens were resistant to HE with essentially no NTS losses. In general, the laser welds exhibited the lower hardness and NTS than the steel plates of the same aging treatment. At the same strength level, the over-aged specimens were far superior to the under-aged ones in terms of HE resistance. This could be associated with the presence of numerous strong traps such as reverted austenite in the over-aged structures to retard hydrogen diffusion to the highly strained region. Conversely, the lack of strong hydrogen traps in the under-aged specimens caused an enhanced susceptibility to HE. It was also noted that the laser welds were susceptible to HE, while the steel plates were insensitive to hydrogen in the peak-aged condition. Apparently, the existence of coarse columnar grains was responsible for a higher NTS loss in hydrogen of peak-aged welds. Fractographic observations generally correlated well with the NTS loss of specimens, i.e. greater NTS losses corresponded to a greater amount of brittle fracture in specimens tested in hydrogen.

Fatigue crack propagation in 2.25 Cr-1.0Mo steel weldments in air and hydrogen

Abstract Fatigue crack growth (FCG) behaviour of 2.25Cr–1.0Mo steel plate and welds tested in air and gaseous hydrogen was investigated. Microstructural observations indicated that both the weld metal (WM) and heat-affected zone (HAZ) were composed of mainly bainitic microstructure. A ferrite matrix with uniformly dispersed carbides was found in the annealed base metal. The change in microstructures of WM and HAZ between the as-welded and tempered welds was limited. The as-welded WM had the lowest impact toughness among the specimens, however, the impact toughness was improved after postweld heat treatment at 700°C/1 h. Regardless of the testing environments, the as-welded WM exhibited a higher resistance to crack growth than the steel plate as the crack propagated transversely to the welding direction, especially in the low ΔK range. However, higher fatigue crack growth rates in the as-welded HAZ than those of the steel plate were found. As the crack grew along the fine-grained heat-affected zone in the as-welded condition, significant retardation of crack growth as compared with that of the steel plate was obtained when tested in air or gaseous hydrogen. In various regions of the weld, the FCG behaviour of tempered welds was similar to that of the steel plate in air. The results also indicated that the tempered bainitic microstructure present in WM or HAZ was more resistant to unstable crack growth than the annealed steel plate tested in gaseous hydrogen. Environmentally induced acceleration of FCG in various regions of the weld was usually exhibited a change of fracture features.

Hydrogen embrittlement susceptibility of laser-hardened 4140 steel

Abstract Slow strain rate tensile (SSRT) tests were performed to investigate the susceptibility to hydrogen embrittlement of laser-hardened AISI 4140 specimens in air, gaseous hydrogen and saturated H2S solution. Experimental results indicated that round bar specimens with two parallel hardened bands on opposite sides along the loading axis (i.e. the PH specimens), exhibited a huge reduction in tensile ductility for all test environments. While circular-hardened (CH) specimens with 1 mm hardened depth and 6 mm wide within the gauge length were resistant to gaseous hydrogen embrittlement. However, fully hardened CH specimens became susceptible to hydrogen embrittlement for testing in air at a lower strain rate. The strength of CH specimens increased with decreasing the depth of hardened zones in a saturated H2S solution. The premature failure of hardened zones in a susceptible environment caused the formation of brittle intergranular fracture and the decrease in tensile ductility.