Novel Development of an NbC-Containing Powder-Metallurgical Martensitic Steel with Outstanding Tribocorrosion Resistance

The development of NbC-containing martensitic stainless steels has made it possible to unite the properties of high corrosion resistance and wear resistance. If NbC is used as a hard phase instead of chromium carbide, the abrasive wear resistance of the steels is increased due to the greater hardness of NbC. The solubility of chromium in NbC is low. For this reason, chromium is fully available to form a passive surface layer to increase the corrosion resistance. In the steel melt, niobium leads to the formation of primary NbC, which grows very rapidly. Atomization of PM steels leads to the formation of coarse primary hard phases that clog the nozzle. Therefore, until now, steels with NbC as a hard phase are produced using the PM route with so-called diffusion alloying; however, this production route is very complex and expensive. Herein, a novel Nb-rich MC-containing wear- and corrosion-resistant steel that is produced using the usual PM route is presented. This steel consists of a martensitic matrix with evenly dispersed Nb-rich MC having a volume of 2.5%. Due to the high hardness (>750 HV30) in combination with high resistance to pitting corrosion, the steel exhibits outstanding tribocorrosion resistance in 0.9% NaCl solution.     

Effect of hydrogen on the fracture behavior of high strength steel during slow strain rate test

The effect of hydrogen on the fracture behavior of the quenched and tempered AISI 4135 steel at 1450 MPa has been investigated by means of slow strain rate tests on smooth and circumferentially-notched round-bar specimens. Hydrogen was introduced into specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS) analysis. Results showed that the steel had high hydrogen embrittlement susceptibility. For both smooth and notched specimens, the fracture mode was changed from microvoid coalescence (MVC) to brittle intergranular (IG) fracture after the introduction of a small amount of diffusible hydrogen. Fracture initiated in the vicinity of the notch root for notched specimens, while it started from around the center in smooth specimens. The fracture stress decreased with increasing diffusible hydrogen content, and the decreasing trend was more prominent for specimens with a higher stress concentration factor. Taking into account the stress-driven hydrogen diffusion and accumulation in the vicinity of the notch root, the local diffusible hydrogen concentration and local fracture stress in notched specimens have been calculated. According to numerical results, the relationship between the local fracture stress and local diffusible hydrogen concentration was independent of stress concentration factor, which could account for the effect of hydrogen on the fracture stress of the steel.     

Creep strength of high chromium steels welded parts under multiaxial stress conditions

Internal pressure creep tests as well as uniaxial creep tests were conducted on 9Cr and 12Cr steels welded joints in order to evaluate creep strength of the high chromium steels welded parts under multiaxial stress conditions, where a welding direction of the internal pressure specimen was longitudinal one. Except under high stress conditions, cracks occurred at fine grained HAZ region for both materials, i.e., the type IV mode fracture. Regarding creep voids distribution along thickness direction of the internal pressure specimens, the creep voids were predominantly observed in the middle of the thickness at the HAZ region while they might be negligible in outer and inner surface of the specimen. Finite element analysis which considers HAZ and weld metal properties revealed that the principal stress (hoop stress) and the stress multiaxiality are the largest in the middle of the thickness at the HAZ region under the internal pressure creep conditions. It was suggested that the creep voids remarkably occurred inside the specimens at the HAZ region due to the severe stress state conditions.     

304不锈钢相变条件下涡流检测频率选取仿真

304奥氏体不锈钢由于其本身组织特性,在制造和在役过程中会产生部分铁素体和马氏体并析出,使其具有一定的磁性,即相对磁导率肼大于1,试验测试结果表明：当不锈钢件形变量在20％以内,随着形变量的增加,试件的磁导率增加,并逐渐开始具有铁磁材料的磁特性,导致不锈钢涡流检测集肤深度降低,也改变了检测的最佳激励频率。此外,通过比较2种不同激励频率的选取方法可得,在不锈钢形变量20％以内,不锈钢形变量增大,其最佳检测频率倾向于降低,且小于100kHz。试验和仿真结果表明,304不锈钢压力容器最佳检测频率范围为40—100kHz。     

High Temperature Corrosion of Chromium–Manganese Steels in Sulfur Dioxide

This work was aimed at explaining the corrosion mechanism of commercial Cr–Mn steels at 1073, 1173 and 1273 K in the atmospheres containing oxygen and sulfur. Three steels were selected for the investigations, two single-phase austenitic steels (Cr17Mn17 and Cr13Mn19SiCa) and a two-phase austenitic-ferritic steel Cr15Mn19. On all studied steels triplex scales were formed. The inner very thin, fine-grained part of the scale contained manganese, chromium and iron sulfides and oxides, the intermediate layer was built mainly of the MnCr₂O₄ spinel while MnO was the predominant constituent of the outer scale layer. According to the gravimetric measurements, after an initial incubation period, the oxidation of steel follows a parabolic rate law. Thermodynamic and kinetic aspects of the formation of oxide-sulfide and oxide layers were discussed. Oxidation was accompanied by depletion of the subscale region of the metallic core in manganese, which is the austenite former. Consequently austenite transformed into ferrite.     

Notch stress analyses of high‐frequency mechanical impact‐improved welds by using ρf = 1 mm and ρf = ρ + 1 mm approaches

This paper presents further assessments of the previously reported round‐robin fatigue data obtained from high‐frequency mechanical impact (HFMI)‐improved longitudinal welds. A detailed statistical analyses of geometry measurements of HFMI‐treated weld toe profiles are presented. The fatigue analyses based on notch stress as defined by the International Institute of Welding are performed using the finite element method. Notch stresses are assessed based on both the fictitious weld toe radius and the addition of measured actual notch radius to the fictitious radius. While no large differences are observed between the results of methods, the former one is found to be more practical and faster to implement from the end‐user point of view.     

Role of alloying elements in nanostructured ferritic steels

Abstract

The roles of the alloying elements in three nanostructured ferritic alloys (14YWT, MA957 and Eurofer 97) have been established through the characterisation of the microstructure by atom probe tomography and spectrum imaging in a transmission electron microscope. Cr, W, Mo, Ti and Y were found in the ferrite matrix and contributed to solid solution hardening. Ti, Y, C, O and N were found in high number densities of precipitates and nanoclusters both in the grain interior and on grain boundaries and thereby contributed to precipitation hardening. Cr, W and Mo were enriched at the intraparticle regions of the grain boundaries. The solute segregation and precipitation pinned the grain boundaries and contributed to the excellent creep properties of the alloys.     

Sintering of ferrous metallic compacts: Chemical reactions that involve interstitial elements

For sintering of ferrous metallic compacts, the chemical reactions with the atmosphere are of special interest. This is due to the very large specific surface of the compacts that makes them highly reactive with the atmosphere, much more than cast and wrought metals. Within this paper, the interactions of interstitial elements with the metallic matrix and the surrounding atmosphere are described. The main reaction partners are interstitial elements that may emerge from the starting powders or from the surrounding atmosphere. The oxidation/reduction and decarburization phenomena have been studied by thermoanalytical techniques both for mixed and prealloyed systems. The reaction mechanisms are discussed as well as their impacts on the sintering processes and the resulting properties. Oxygen transfer reactions (“Internal gettering”) and formation of methane are observed for systems with widely varying oxygen affinity of the components. The results presented show that controlling the interactions of the interstitial elements with the metallic components is crucial for successful sintering of advanced PM alloy steels.     

Ermittlung von Umformgrenzen beim Freiformschmieden

Determination of forming limits during open die forging The fundamentals of determining the forming limits during open die forging have been investigated in the research project “Vermeidung von Oberflächenfehlern beim Freiformschmieden” (“Prevention of surface defects during open die forging processes”) which was supported by the AiF (“Arbeitsgemeinschaft industrieller Forschungsvereinigungen”). The industrially produced material was analysed in the form of cast ingots of two high‐alloyed steels (1.2367 and 1.6957) by compression, tension and torsion tests. The tests showed that the sampling point has a small influence on the materials’ plasticity for the present dimensions of the ingots. Furthermore transformation and precipitation behaviour of the both steels have been determined and additional metallographic investigations have been made to find out reasons of variation of measurements for low forming temperatures. Forming limit diagrams are generated to predict the forming limits of open die forging processes. For this purpose, compression tests and numerical simulations of the compression tests and forging processes were made for crack‐critical temperatures. The points in time of crack initiation of the samples during the compression tests were determined by the acoustic emission analysis and show a high variation of measurements. Thus the forming limit diagrams also have high scatter, an unique forming limit cannot be determined. Instead of that a maximum allowed height‐reduction was determined and checked in open die forging tests in the laboratory under industrial conditions. The results show that the determined limits are very safe because no cracks were generated during the forging of both materials.     

Nb对转K2弹簧钢中MX析出相的影响

采用Thermo-Calc热力学软件,对转K2弹簧钢在400~1600℃存在的平衡析出相及Nb、V在奥氏体相中的固溶规律进行了计算,探讨了各合金元素含量对析出相析出规律的影响,并利用碳萃取复型与透射电镜(TEM)分析了Nb含量对析出相的影响.结果表明:转K2弹簧钢中平衡析出相主要为MX、M₇C₃、M₃C₂和AlN;Nb能显著提高MX相的稳定性.TEM分析表明,析出相尺寸变化范围为几纳米到100纳米以上,形态多呈近似球形或圆片状;随Nb含量增加,从低温铁素体中析出的细小颗粒所占比例显著增大,未溶解碳氮化物比例也有所增大,总体而言析出相平均颗粒尺寸得到细化;大颗粒析出相为富Nb的碳化物,而小颗粒为富V的碳化物,这与热力学计算结果相一致.