Takeshi Kuwana

Effect of nitrogen on impact toughness of duplex stainless steel weld metal

Summary A type 329J1 duplex stainless steel was welded by single‐pass gas tungsten arc welding in an argon‐nitrogen mixed gas atmosphere. The microstructure, Vickers hardness, and Charpy impact toughness of the weld metal were examined. The nitrogen content increases and the ferrite content decreases with increasing nitrogen partial pressure of the atmosphere. The ferrite content linearly decreases with an increasing nitrogen content. The hardness of the ferrite and austenite phases is virtually constant regardless of the weld metal nitrogen content. The toughness increases with the nitrogen content at room temperature and above, remaining constant at lower temperatures. The weld metal toughness is affected by the presence of austenite and chromium nitride precipitates in the ferrite. The nitrogen absorption of duplex stainless steel weld metal improves the impact toughness through an increase in the austenite content and a decrease in the amount of nitride.

Effects of nitrogen on tensile properties of duplex stainless steel weld metal

Summary Type 329J1 duplex stainless steel was welded by gas tungsten arc welding in argon‐nitrogen mixed gas atmospheres. The tensile properties and microstructures of the weld metals were examined. The nitrogen content increases and the ferrite content decreases with increasing nitrogen partial pressure of the atmosphere. The ferrite content linearly decreases with an increasing nitrogen content. The tensile strength and elongation of the weld metal produced in the argon atmosphere are much lower than those of the base metal, but they increase with an increasing nitrogen content and approximate those of the base metal at around 0.4 mass% nitrogen content. The fractographs suggest that only the base metal and high‐nitrogen weld metal clearly show dimple patterns. The tensile‐tested base metal and high‐nitrogen weld metal have complex crack paths, whereas the other weld metals have relatively straight paths. The tensile properties of the weld metal are affected by the ferrite content and chromium nitride.

Dependence of Carbide Precipitation on Grain Boundary Structure in Sensitized Austenitic Stainless Steel

Grain boundary carbide precipitation and intergranular corrosion in sensitized austenite stainless steel were examined by transmission electron microscopy (TEM) to clarify the effect of grain boundary structure on precipitation and corrosion. A type 304 steel, which had been solutionized at 1350 K was heat-treated at temperatures of 800-1300 K. Oxalic acid etch and Strauss tests showed that the frequency of grain boundaries with M23C6 carbide precipitation and corroded boundaries increased with holding time at sensitizing temperatures. The grain boundary carbide precipitation was observed during heat treatment at 1000 K by TEM. Grain boundaries were characterized on the basis of the Coincidence Site Lattice (CSL) theory using electron diffraction Kikuchi patterns. The observations revealed that the propensity to intergranular precipitation depends strongly on the grain boundary structure. Carbide precipitates tend to be detected at grain boundaries with higher Σ -values or larger deviation angles (Δθ) from low- Σ CSL misorientations. The border lines between precipitation and no precipitation can be drawn by a deviation parameter of Δθ/ΔθC, where Δθc is the maximum deviation angle by Brandon’s criterion. The border line of Δθ/Δθc decreased with the increase in the holding time at 1000 K. This means that the more ordered boundary needs the longer time for intergranular carbide precipitation and corrosion than less ordered or random boundaries.

Effect of nitrogen on transformation temperatures of arc-melted, Ti-Ni shape memory alloys

Summary This paper describes an investigation into the effect of nitrogen on the microstructures and phase transformation temperatures of near-equiatomic, Ti-Ni shape memory alloys arc-melted in Ar-N2 atmospheres. The nitrogen content of the arc-melted samples increases with an increasing melting time and rising nitrogen partial pressure in the atmosphere (arc current of 150 A, arc voltage of 12 V, sample weight of 12 g and melting time of 0–480 sec). The amount of second phase observed in the arc-melted samples increases with an increasing nitrogen content. The second phase is identified as TiN by electron probe microanalysis (EPMA) and X-ray diffraction analysis. Both the Ms and Af temperatures decrease with the increasing nitrogen content of the arc-melted samples. The Ms and Af temperatures are described by functions of the nickel and nitrogen contents.

Oxygen absorption by pure iron weld metal in gas tungsten arc welding

Summary Pure iron plate was welded by the gas tungsten arc welding process in an argon‐oxygen gas mixture. The oxygen absorption behaviour of the pure iron weld metal is metallurgically investigated. The oxygen content of the weld metal decreases with an increasing welding current and an increasing arc length. The travel speed has only a minor effect on the oxygen content of the weld metal. The oxygen content of the weld metal sharply increases and then gradually increases with a rising oxygen partial pressure. In the high oxygen partial pressure domain, the oxygen absorption behaviour is unlike that in gas metal arc welding. This difference may well be due to the slowdown of the oxygen absorption rate in gas tungsten arc welding.

Welding Current and Nitrogen Content of Weld Metal (Report 1)

Effect of welding current and sampling method on the nitrogen content of weld metals in nitrogen and air atmospheres using a pure iron electrode wire was studied.Three methods i.e. depositing on a mild steel plate, rotating copper disk and water cooled copper block were used for sampling at different current levels.The main results obtained are summarized as follows;1. The nitrogen content of weld metals obtained by the bead on mild steel plate method decreases slightly in a nitrogen atmosphere but largely in an air atmosphere with increasing welding current.2. The rotating copper disk method is not suitable for the study of nitrogen absorption as the weld metal droplets are not so rapidly chilled in water bath and the arc condition is pretty different from that of the conventional arc welding.3. The water cooled copper block method is more suitable than the rotating copper disk method for studying nitrogen absorption through arc space, though it can not be used at the ionhigher currents.4. The head on plate method is useful for studying the absorption of nitrogen by molten droplets and weld pool at low welding current.