Quantification of the Post-Fire Strength Retention Factors for Selected Standard Duplex and Lean Duplex Stainless Steel Grades.

The experimental quantification of retention factors related to the post-fire strength as well as the post-fire ductility of intentionally selected stainless steel grades applied in construction is the objective of the research presented here. These steel grades are characterized by a two-phase austenitic-ferritic microstructure of the duplex type. In this context, two mutually corresponding chromium-nickel-molybdenum steel grades are subjected to analysis, namely X2CrNiMoN22-5-3 steel belonging to the standard duplex group (DSS 22% Cr) and X2CrMnNiN21-5-1 steel belonging to the lean duplex group (LDSS). The similarities and differences in the mechanical properties exhibited by these steel grades after effective cooling, following more or less prolonged simulated fire action conforming to several development scenarios, are identified and indicated. The resistance of a given steel grade to permanent structural changes induced by the heating program proved to be the critical factor determining these properties and resulting in many cases in increased susceptibility to brittle fracture. The results obtained experimentally seem to confirm the quantitative estimates of post-fire retention factors forecast by Molkens and his team, specified for the steels exhibiting a duplex-type structure and tested by us. However, several of these estimates might be considered somewhat risky. Nevertheless, our results do not confirm the significant post-fire strengthening of steel grades belonging to the LDSS group following prior heating at a sufficiently high temperature, as reported earlier by Huang Yuner and B. Young.

Kinetics of Intermetallic Phase Precipitation in Manual Metal Arc Welded Duplex Stainless Steels.

The article presents the influence of heat treatment on the kinetics of transformations in lean duplex LDX2101 steel and a weld made of standard duplex 2209 material, which was welded by manual metal arc welding. Changes in the microstructure, hardness, and magnetic phase content were analyzed after heat treatment was conducted at a temperature of 800 °C for a period ranging from 15 to 1440 min. Light and scanning microscopy, Vickers hardness measurements, and magnetic phase content measurements using a ferritoscope were used for the research. In the LDX2101 steel, the presence of δ-ferrite and γ austenite was identified and additional Cr2N nitrides were observed in the heat-affected zone. After heat treatment, the decomposition of δ ferrite into γ2 austenite and Cr2N nitrides was observed in both areas. In the case of weld made by the coated electrode in 2209 grade, a ferritic-austenitic microstructure with allotriomorphic austenite (γA), Widmanstätten austenite (γW), and idiomorphic austenite (γI) and δ-ferrite area with "bee swarms" of fine precipitations of chromium nitrides Cr2N and non-metallic inclusions (NMIs) of slag, formed during the welding process, are observed in the as-welded state. After heat treatment, the presence of the χ phase (after 15 min of annealing) and the σ phase (after 120 min of annealing) was additionally identified. The kinetics of intermetallic phase evolution in welds made from 2209 material were presented. The obtained results of hardness measurements and metallographic tests were correlated, which allowed for a quick check of the precipitation processes on the used element.

Influence of Long-Term Subcritical Annealing on the Unalloyed Steel Welded Joint Microstructure.

The article presents changes in the microstructure of hot-rolled unalloyed structural steel after the arc welding process and in the state after long-term exposure to 600 °C during operation. These studies enable a clear assessment of the effects of long-term exposure to elevated temperature relative to the as-welded condition, which has not been reported. The microstructure examination was carried out on welded joints in eight different zones of the joint. Studies have shown that the welding thermal cycle causes significant changes in the microstructure in the area of the base material heated above the A1 temperature-the heat-affected zone (HAZ)-and in the weld area in the case of multi-pass welding. The long-term exposure of the subcritical temperature of 600 °C on the welded joint leads to the phenomenon of cementite spheroidization in the pearlite in all zones of the joint, while preserving the band structure of the steel after rolling and the structural structure. In the case of the weld, acicular and side-plate ferrite disappearance was observed.

Preliminary Process and Microstructure Examination of Flux-Cored Wire Arc Additive Manufactured 18Ni-12Co-4Mo-Ti Maraging Steel.

The production of large-size elements using additive manufacturing is a constantly evolving field that includes technological and material solutions. There is a need for a detailed analysis of the process and the products thus manufactured. In line with this trend, the flux-cored wire arc additive manufactured process and the part made of 18Ni-12Co-4Mo-Ti maraging steel were examined. The interpass temperature below 150 °C, the variation of the starting point and the gas flow of 12 L/min with a pre-flow of 2 s ensure the correct shape of the layers. The manufactured part underwent chemical composition analysis, macro- and microscopic examination and hardness measurements; in addition thermodynamic calculations were performed. The part is divided into a light-etched area (bottom part of the sample) with a hardness of 375 ± 12 HV10 and a dark-etched area (top part of the sample) with a hardness of 525 ± 11 HV10. Microscopic observations in the last layers showed supersaturated martensite with primary precipitates of µ-phase intermetallic compounds in intercellular spaces. In the earlier layers aging martensite with austenite and primary precipitates of intermetallic compounds were revealed. The share of austenite was 11.435 ± 1.313%.

Influence of the Thermal Cutting Process on Cracking of Pearlitic Steels.

The paper presents research results of the influence of heat input into high carbon rail steel during cutting processes on microstructure transformation and cracking. The massive block of steel prepared for rail rolling processes was cut and examined by nondestructive magnetic testing and destructive testing by microscopic examination and hardness measurements. The results show unfavorable microstructure changes where pearlite and transformed ledeburite were obtained. The effects of the presence of such microstructures are high hardness near to cutting surfaces (above 800 HV) and microcracks which grow into low hardness block cores during rolling and rail shaping.

The Wear on Roller Press Rollers Made of 20Cr4/1.7027 Steel under Conditions of Copper Concentrate Briquetting.

This paper defines the wear process of rollers made of 20Cr4. Rollers with a diameter of 1000 mm were installed in a roller press used for the production of drop-shaped briquettes and the copper concentrate was briquetted for 1100 h. Three-dimensional (3D) geometry analysis, metallographic analysis, macroscopy, scanning electron microscopy, as well as hardness measurements were performed. It was observed that the working surface was non-uniformly worn. The smallest wear affects the molding cavities situated on the outermost edges of the ring. The wear increases as the center of the ring is approximated, and it reaches its maximum at the middle of the ring. The molding cavities also wear asymmetrically. For the shape considered in this study, the lower part of a cavity is subject to a higher wear rate. We found that the material of the working ring was carburized, but its hardness was significantly lower than required. The roller ring microstructure changes depended on the distance from the cavity's face. An investigation of the wear mechanisms showed different types of abrasive wear, corrosive processes, and plastic deformation. The exact type and course of wear were described, depending on the location on the working surface.

Laser Dissimilar Welding of AISI 430F and AISI 304 Stainless Steels.

A dissimilar autogenous laser welded joint of AISI 430F (X12CrMoS17) martensitic stainless steel and AISI 304 (X5CrNi18-10) austenitic stainless steel was manufactured. The welded joint was examined by non-destructive visual testing and destructive testing by macro- and microscopic examination and hardness measurements. With reference to the ISO 13919-1 standard the welded joint was characterized by C level, due to the gas pores detected. Microscopic observations of AISI 430F steel revealed a mixture of ferrite and carbides with many type II sulfide inclusions. Detailed analysis showed that they were Cr-rich manganese sulfides. AISI 304 steel was characterized by the expected austenitic microstructure with banded δ-ferrite. Martensitic microstructure with fine, globular sulfide inclusions was observed in the weld metal. The hardness in the heat-affected zone was increased in the martensitic steel in relation to the base metal and decreased in the austenitic steel. The hardness range in the weld metal, caused by chemical inhomogeneity, was 184-416 HV0.3.