Carburization kinetics of heat-resistant steels

A series of 21 commercial austenitic heat-resistant alloys was carburized in H₂-C₃H₆ atmospheres which were reducing to Cr₂O₃ and SiO₂ and which provided a carbon activity of one. Reactions were carried out at 900T1100°C and the resulting depth of carburization was measured metallographically. All alloys showed parabolic carburization kinetics after an initial brief period, at low temperatures, of more rapid reaction. The apparent activation energies for carburization are discussed in terms of carbon solubilities and diffusion coefficients. It is concluded that Wagner's theory of internal oxidation is quantitatively applicable to simple alloys but not to alloys which contain additions of reactive elements. The effectiveness of additions of niobium, aluminum, titanium, and silicon is demonstrated and is discussed in the context of Wagner's theory.     

Corrosion of carbon steels in high-temperature water studied by electrochemical techniques

Electrochemical methods were used to study the corrosion behavior of five Fe-Cr alloy steels and 304L stainless steel in high-temperature water. Passivity can be achieved on A-106 B carbon steel with a small content of chromium, which cannot be passivated at room temperature. The formation rate and the stability of the magnetite film increased with increasing Cr content in the steels. A mechanistic model was developed to describe the corrosion processes of steels in high-temperature water. The crack growth rate on steels was calculated from the maximum current of the repassivation curves according to the slip-oxidation model. There was a high crack growth rate on 304L stainless steel in high-temperature water. Of the four Fe-Cr alloys, the crack growth rate was lower on 0.236% Cr- and 0.33% Cr-steels than on 0.406% Cr steel and 2.5% Cr-1% Mo steel. The crack growth rate on 0.33% Cr steel was the smallest over the tested potential range.An increase in Cr content in the steel is predicted to reduce the corrosion rate of steel at high temperatures. However, this increase in Cr content is predicted not to reduce the susceptibility of steel to cracking at high temperatures.     

The corrosion wear behaviour of selected stainless steels in potash brine

Tests were conducted at 25 and 85 °C to evaluate the corrosion wear resistance of selected stainless steels in potash brine using a reciprocating motion wear apparatus. Four materials were tested: Ferralium 255 (UNS S32550), AL6XN (UNS N08367), 254SMO (UNS S31254) and AISI 1018 (UNS G10180) for comparative purposes. The evaluation methods employed included weight loss analysis, optical microscopy and scanning electron microscopy (SEM). The results show that Ferralium 255 has superior corrosion wear resistance in potash brine environment compared to AISI 1018 plain-carbon steel and the other stainless steels tested. Wear surface analysis using SEM shows evidence of brittle fracture damage, which is attributed to the presence of Cl⁻.     

Corrosion behavior of AISI302 steel treated by elevated temperature nitrogen plasma immersion ion implantation

The corrosion behavior of AISI302 steel implanted with nitrogen at elevated temperature was investigated by electrochemical impedance spectroscopy. Equivalent circuits for explaining the impedance characteristics are proposed. The thick passive layer containing Cr₂O₃ and the expanded austenite layer in the sub-surface worked together, resulting in the high corrosion resistance.     

High Carbon Alloy Steels with Multiple Types of Ultra-fine Carbides and Their Characteristics

Under normal forging and annealing conditions, there are different ultra fine carbides (M3C, M23C6, M7C3, M6C and MC) in high carbon alloy steels when alloy composition design is carried out properly. On the basis of carbides transformation orderliness, the alloy composition design of the high carbon alloy steels is conducted by phase-equilibrium thermodynamic calculation for Fe-Cr-W-Mo-V-C system. The nucleation and growth of new carbides, dissolution of previous partial carbides in these steels during annealing process, all these lead to ultra-fine distribution of carbides. Due to different crystal structures of carbides and different thermodynamics as well dynamics parameters of the carbides dissolution and precipitation, the range of quenching temperature of these steels is widened, and the good temper-resistance is obtained. The characteristics of heat treatment process and microstructure variance, and the carbides transformation for different temperature are explained by the phase-equilibrium component satisfactorily. Their bend and yield strength,flexibility and toughness all are advanced markedly comparing with that of kindred steels. Results of the applications have proved that the microstructure of ultra-fine carbides in these steels played importance roles in the enhancement of edginess and fatigue crack resistance of the die and knives.     

Liquid metal embrittlement susceptibility of T91 steel by lead–bismuth

Previous studies on T91 steel in its standard metallurgical state do not provide evidence for liquid metal embrittlement by eutectic Pb–Bi. In this paper, we show that this steel can be embrittled by Pb–Bi when direct contact between the steel and the liquid metal is obtained by prior ion beam sputtering of the native oxidized film.     

INTRAGRANULAR FERRITE FORMED IN ASSOCIATION WITH INCLUSIONS IN A VANADIUM MICROALLOYED STEEL

lntragranular ferrite was formed at inclusions in a vanadium microalloyed steel with excess amount of sulfur. The chemical composition of inclusions in the steel was analyzed by SEM-EDS. The inclusions were mainly composed of MnS and aluminum oxides. The precipitation of MnS at aluminum oxides might result in Mn depletion, which, in turn, promotes the formation of intragranular ferrite. Optical and SEM observations and threedimensional (3D) reconstruction demonstrated that intragranular ferrite was formed at inclusions. The morphology of intragranular ferrite changed with undercooling. At higher temperatures intragranular ferrite was nearly equiaxed whereas it was plate-like or lath-like at lower temperatures.