Mechanical Properties Of Martensitic Steel Research Articles

Residual stress evaluation in innovative layer-level continuous functionally graded materials produced by Powder Bed Fusion-Laser Beam

The main objective of this study is to evaluate residual stresses in AISI 316L and 18Ni Maraging 300 functionally graded materials with continuous variation of composition within a single layer using the contour method. The manufacture of this kind of layer-level continuous functionally graded materials by employing a Powder Bed Fusion-Laser Beam system utilizing a blade/roller-based powder spreading technique has only been recently devised and a proper residual stress analysis is still required. In fact, as the mechanical properties of additively manufactured samples are significantly influenced by the direction of construction, the same holds true for the direction along which the compositional variation is made. Furthermore, in this study, the impact of solution annealing and aging heat treatment, which are necessary for enhancing the mechanical properties of martensitic steel, on residual stresses was explored. Additionally, the effect of adopting material-differentiated process parameters was investigated. The results indicated that each specimen displayed areas of tensile stress concentration on the upper and lower surfaces, balanced by compression in the center. The application of heat treatment led to a decrease in the maximum tensile stress of 8% and provided a uniform and significant stress reduction within the maraging steel. Finally, the implementation of material-specific process parameters for the three composition zones in conjunction with the heat treatment resulted in a reduction in the maximum residual stress of 35% and also a significantly lower residual stress field throughout the specimen.

Distribution behavior of inclusions in compact-strip-produced martensitic steel and its influence on mechanical properties

The distribution behavior of inclusions in martensitic steel produced by compact strip production process (CSP-MS) and its influence on mechanical properties were systematically investigated. The inclusions in the CSP-MS specimen are mainly composed of spherical Al2O3-CaO-CaS, MnS with high aspect ratio and small-sized TiN, whereas many coarse cuboidal TiN inclusions do exist in conventional martensitic steel (Con-MS). The high inclusion density of the CSP-MS specimen resulted in lower total elongation and impact toughness, and the MnS inclusions with high aspect ratio led to significantly stronger mechanical anisotropy than that for Con-MS specimen. The in-situ tensile results indicated that when the fracture direction is parallel to MnS direction, the microcracks induced by MnS inclusions tend to propagate into the matrix, leading to the formation of valley-like features, which significantly deteriorate the properties such as the total elongation and impact toughness. The microcracks caused by TiN inclusions are sharper than those caused by spherical Al2O3-CaO-CaS inclusions, and are easy to propagate into the matrix. This work is expected to guide the optimization of the mechanical properties of martensitic steels produced by CSP process and provide a theoretical basis for the CSP process design.

MECHANICAL PROPERTIES OF P91 AND PB2 STEEL AFTER LONG-TERM AGEING AT 620°C

The article presents the results of research on mechanical properties of martensitic steels, X10CrMoVNb9-1 (P91) and X13CrMo- CoVNbNB9-2-1 (PB2), as-received and after 50,000 hours of ageing at 620˚C. The scope of the tests of mechanical properties included a Vickers hardness test, Charpy impact test, and static tensile test. As received, the investigated steels were characterised by relatively high mechanical properties. The long-term effect of temperature and time contributed to a relatively slight decrease in the strength properties and hardness of the tested steels. However, a considerable decrease in the ductility of these alloys was observed. The decrease in mechanical properties after long-term ageing was smaller in the case of the PB2 steel, which was attributed to the beneficial effect of microalloying boron.

Internal residual stress originated from Bain strain and its effect on hardness in Fe–Ni martensite

To further understand the internal residual stress that is microscopically generated via martensitic transformation in steels, the origin of internal strain attributed to Bain correspondence between face-centered cubic (fcc) and body-centered cubic (bcc) was evaluated from macro- and micro-viewpoints and its effect on hardness was investigated in an interstitial free Fe-16%Ni martensite. Neutron diffractometry and electron backscatter diffraction analysis showed that body-centered cubic (bcc) crystal structure of as-quenched martensite contained elastic distortions leading to small tetragonality, even in martensitic steels without solute carbon, and that the extended [001]bcc of martensite tended to be parallel to <001>fcc of prior fcc austenite. In addition, the combination of micro-scale focused ion beam (FIB) and high-precision digital image correlation techniques revealed that a micropillar fabricated by FIB processing within a martensite block was anisotropically deformed by the release of the residual strain distributed in as-quenched martensite in correspondence with the tetragonal distortions of the bcc crystal structure. These results prove that a small part of the Bain strain remained as an internal elastic residual strain and was microscopically distributed among Bain groups in lath martensite after martensitic transformation. Furthermore, the residual strain generated a hydrostatic internal stress, and therefore, the nanohardness decreased considerably by the micropillar fabrication accompanied by the release of the internal stress. This means that the internal residual stress in martensite among Bain groups influences the mechanical properties of martensitic steel.

Internal Residual Stress Originated from Bain Strain and its Effect on Hardness in Fe-Ni Martensite

To further understand the internal residual stress that is microscopically generated via martensitic transformation in steels, the origin of internal strain attributed to Bain correspondence between face-centered cubic (fcc) and body-centered cubic (bcc) was evaluated from macro- and micro-viewpoints and its effect on hardness was investigated in an interstitial free Fe-16%Ni martensite. Neutron diffractometry and electron backscatter diffraction analysis showed that the as-quenched martensite had a body-centered tetragonal (bct) crystal structure with small tetragonality, even in martensitic steels without solute carbon, and that the [001]bct of bct martensite tended to be parallel to fcc of prior fcc austenite. In addition, the combination of micro-scale focused ion beam (FIB) and high-precision digital image correlation techniques revealed that a micropillar fabricated by FIB processing within a martensite block was anisotropically deformed by the release of the residual strain distributed in as-quenched martensite in correspondence with the orientation of the bct crystal structure. These results prove that a small part of the Bain strain remained as an internal elastic residual strain and was microscopically distributed among Bain groups in lath martensite after martensitic transformation. Furthermore, the residual strain generated a hydrostatic internal stress, and therefore, the nanohardness decreased considerably by the micropillar fabrication accompanied by the release of the internal stress. This means that the internal residual stress in martensite among Bain groups influences the mechanical properties of martensitic steel.

Study of deformation behaviors of martensitic steel quenched at ultralow temperature

A sandwiched structure made of alternate layers of martensitic and austenitic steel has been quenched in both water and liquid nitrogen, and their deformation behaviors have been investigated via uniaxial tensile loading with an elongation of more than 40%. It demonstrates that quenching temperature, compared with the large-strain tensile deformation, has an even more significant influence on the improvement of the mechanical properties of martensitic steel.

Analysis of the microstructure and properties of T92 steel after long-term service

Purpose: The purpose of the investigation was to determine and analyse the changes in the microstructure and mechanical properties of the T92 steel after service in creep conditions of the following parameters: temperature – 575°C, pressure – 28.2 MPa, service time – 41914 hrs. Design/methodology/approach: The tests were performed on the test samples taken from a pipe section of a steam superheater after long-term service. The range of the investigations included: microstructural investigation – the optical and SEM microscopy, the analysis of precipitation – carbide isolates, the investigation of mechanical properties: the Vickers hardness measurement, the impact test and static tensile test. Findings: The performed tests showed a slight degree of exhaustion of the structure of the analysed T92 steel. The relatively small changes in the microstructure of the examined steel were reflected in the still retained high mechanical properties. Research limitations/implications: he analysis of the microstructure of the examined steel using SEM was performed to determine the influence of the service on the processes of changes in the precipitate morphology. Practical implications: The metal science investigation of the sections taken from the elements of the power installations after long-term service is one of the basic elements of building the data base of materials and their joints used in the power industry. The results obtained from the performed research constitute a building block for the degradation characteristics of the microstructure and mechanical properties of martensitic steels of the 9-12%Cr type. Originality/value: The results of investigation and analysis of the metallographic and mechanical properties of martensitic T92 steel after long-term service are presented.

Effect of grain refinement on mechanical properties of martensitic steel

Microstructure and strength of commercial martensitic steel processed by equal channel angular pressing (ECAP) has been investigated using transmission electron microscopy and tensile tests. Application of ECAP led to significant grain refinement to a grain size of 0.8 µm and precipitation of particles with a size of 90 nm. Impact of annealing on thermal stability of ultrafine-grained structure was investigated. The contribution of various strengthening mechanisms to strength of steel was discussed.

Heat treatment conditions to prevent failure in die cast X38CrMoV5 steel parts

The usual mechanical properties of martensitic steels are strongly linked to the complex microstructure obtained after their heat treatment, which is generally performed to achieve good hardness and/or tensile strength with acceptable ductility. In the present work, X38CrMoV5-1 (AISI H11-1) steel, classed as a hot-work tool steel with many applications, has been studied from the viewpoint of phase transformations and heat treatment. Parts are often manufactured by die-casting followed by quenching and tempering and the microstructure is highly sensitive to the process. Microstructural defects can lead to early failure of many parts at the service temperature as a consequence of the casting process and heat treatment conditions. The authors have used dilatometry and differential thermal analysis (DTA) techniques to study phase transformation during cooling and have determined CCT diagram. Heat treatment is not able to remove the casting dendritic microstructure or porosity, thus increasing the likelihood of brittle fracture, but it can prevent catastrophic failure in service conditions.

Effect of tempering temperature on isothermal decomposition product formed below Ms

Recent papers have shown that bainitic ferrite plates can be produced by transformation at low temperatures, resulting in enhanced mechanical properties of martensitic steels and increased resistance to tempering. To that end, experimental investigations were performed on the formation of the microstructure in SAE 9254 steel during isothermal heat-treatment below the martensite start (Ms) temperature. After the isothermal heat-treatment, the same steel was subjected to tempering at various temperatures. Vickers microhardness measurements were performed on the isothermally heat-treated and tempered samples, and microhardness values for the samples were compared with those for quenched and tempered samples of the same steel. All the isothermally treated samples contained the martensite (αμ), bainitic ferrite (αb), and retained austenite (γr) phases. The retained austenite blocks (γb) decomposed in the samples tempered at 400°C. A significant decrease in the microhardness value for the sample heat-treated at 270°C can only be observed after the sample was tempered at 450°C, because bainite was the predominant phase in the microstructure, which self-tempers as it forms.

Effects of Austenitising Conditions on the Microstructures and Mechanical Properties of Martensitic Steel with Dual Matrix Structure

A new high strength steel with dual matrix structure and exceptionally high toughness plus ductility have been produced by intermediate quenching of 0.22_wt% C microalloyed steel. The treatment consisted of initial austenitization and rapid quenching of the steel samples to achieve a fully martensitic state followed by annealing in the intercritical (α+γ) region of 730℃-810℃ for the period of 30, 60 and 90_minutes. These samples were subsequently quenched to obtain dual phase microstructure containing varying proportions of ferrite and martensite constituents. The mechanical properties of the samples were measured according to ASTM standard and their microstructures were analyzed by optical microscopy. The experimental results show that martensitic dual phase (MDP) steel samples developed within the intercritical temperature range of 770–790℃ revealed finer martensite and precipitate-free ferrite microstructure. The tensile and impact properties of the developed HMDP steels increased with intercritical annealing (ICA) temperatures, with an optimum properties obtained at 790℃ mainly due to finer microstructure of the constituent phases and absence of carbide precipitate that permit ease of dislocation flow. A further increase in the intercritical annealing temperature beyond 790℃ led to general decrease in the mechanical properties. This is attributed to the formation of coarse structure in this region. The results further show that with increasing intercritical treatment time from 30 to 90 minutes, the general mechanical properties of the MDP steels were found to increase except at the higher temperature of 810℃ which showed decreasing values. In general, the tensile strengths and ductility as well as the impact properties of the developed dual phase steel samples are greatly improved with the intercritical heat treatment investigated.

Mechanical properties of martensitic steels in gaseous hydrogen

We present the regularities of hydrogen degradation of 03Kh12N10MT, 15Kh12N2MFAV and 13Kh11N2V2MF steels under a pressure of 30 MPa within the temperature range of 293–673 K. The minimum values for plasticity, low-cycle fatigue, and static crack resistance, which do not decrease with an increase in pressure of hydrogen atmosphere and content of the absorbed hydrogen, are found. The difference between temperature dependences of the coefficients of influence of hydrogen on static and cyclic crack resistance of martensitic steels with various content of austenite is established. The main fractographic features of the influence of hydrogen on the micromechanism of fracture of steels under different types of loading and temperatures are determined.

Effects of Deformation Strains and Annealing Temperatures on Mechanical Properties of Martensitic Steels

The effects of deformation strains and annealing temperatures on microstructures and mechanical properties of martensitic steels were examined. The amount of cold deformation was changed as 30%, 50% and 60%, and annealing temperatures varied from 500°C to 600°C. In samples cold rolled 30%, the dominant microstructure for an annealing at 500°C was dislocation substructures with uniformly distributed rod-shaped carbide particles. For an annealing at 600°C, the microstructure consisted of equiaxed ultrafine grains, spherical carbide particles and elongated dislocation substructures. A proper annealing temperature for martensitic steels received 30% reduction, showing a good combination of a high strength, 1230MPa, and an adequate total elongation. 9.4%, was found as 500°C.

Formation of Ultrafine-grained Structure at Drill-hole Surface of Martensitic Steels by High-speed Drilling and Their Mechanical Properties

Microstructure and mechanical properties of martensitic steels after ultrafine-crystallized drilling were investigated. Nano- (at top surface) and submicron-crystalline structures (UFG structure) were formed near drill-hole surface at more than Ac1 by ultrafine-crystallized drilling. The nanocrystalline structure has extremely high hardness and high thermal stability. These characteristics of nanocrystalline structure were similar to those formed by ball milling and shot peening. The tensile property of specimen with UFG structure near the specimen surface showed the similar deformation behavior to that without UFG structure. The fatigue life of specimen with surface UFG structure increased independently of the existence of residual compression stress as compared to that without UFG structure.

Mechanical properties of martensitic steels after exposure to flowing liquid metals

This paper describes the tensile test results of martensitic steels EUROFER 97 and T91 after exposure to flowing Pb–17Li and Pb–55.5Bi alloy, respectively. The corrosion tests were performed in LIFUS II loop (Pb–17Li) and LECOR loop (Pb–55.5Bi) located in the ENEA Brasimone Center. Tensile test were carried in SYNTECH test machine under Ar atmosphere with an extension rate of 2 mm/min, and test temperature for EUROFER and T91 steels were at 480 and 400 °C respectively. The exposure of EUROFER steel to the liquid Pb–17Li did not affect its mechanical properties, while the ductility of T91 steel was deteriorated after exposed to flowing liquid Pb–55.5Bi.

Mössbauer studies of medium-carbon, high-chromium martensitic steels

57Fe Mossbauer effect spectroscopy is employed to determine the relationship between the microstructure and the mechanical properties of martensitic steels with base composition Fe-10wt%Cr-0,26wt%C. The microstructure consists predominantly of two phases: martensite and austenite. The effect of low concentrations of both Mn and Ni on the structure and the mechanical properties of these steels is studied. The results indicate that Mn and Ni additions are equally effective in increasing the fraction of retained austenite. The austenite is an important phase since it is considered to be beneficial to the toughness of steel. However, we find that the impact toughness first decreases and then increases as a function of the fraction of austenite.