Mixed Structure Of Martensite Research Articles

Effect of martensite morphology on mechanical properties of low alloy steels having mixed structure of martensite and lower bainite

AbstractTwo low alloy structural steels having carbon contents of 0·2 and 0·4 wt-% and similar levels of Cr, Mo, and Ni have been studied to determine the effect of martensite morphology on mechanical properties of steels having a mixed structure of martensite and lower bainite. The 0·2% and 0·4%C steels were isothermally transformed at 693 and 593 K, respectively. In the 0·2%C steel, the lower bainite was acicular inform and partitioned the prior austenite. The martensite formed on quenching was of entirely lath morphology. The transformation products of the 0·4%C steel were similar, but a small amount of twinned martensite was also present. After tempering at 473 K, the values of strength, fracture ductility, and true notch tensile strength of the 0·4%C steel were considerably superior to those of the 0·2%C steel. This difference resultsfrom plastic restraining of the lower bainite by the stronger martensite of the 0·4%C steel during plastic deformation. The details are discussed in terms of the modifie...

Effect of microstructure on plane-strain fracture toughness of aisi 4340 steel

Commercially available AISI 4340 steel has been studied to determine the effect of transformation structures on plane-strain fracture toughness (K IC). Martensitic and bainitic steels with wide variation in the prior austenitic grain size, and steels having two different mixed structures of martensite and bainite were investigated. Microstructures were examined by optical and transmission electron microscopy. Fracture morphologies were characterized by scanning electron microscopy. The significant conclusions are as follows: in a martensitic or lower bainitic steel in which well-defined packets were observed, the packet diameter is the primary microstructural factor controllingK IC. The steel's property is improved with increased packet diameter. If the steel has an upper bainitic structure, the packet is composed of well-defined blocks, and the block size controls theK IC property. When the steel has a mixed structure of martensite and bainite, the shape and distribution of the second phase bainite have a significant effect on theK IC property. A lower bainite, which appears in acicular form and partitions prior austenite grains of the parent martensite, dramatically improves theK IC in association with tempered martensite. If an upper bainite appearing as masses that fill prior austenite grains of the parent martensite is associated with tempered martensite, it significantly lowers the KIC.

Mechanical properties of 0.40 pct C-Ni-Cr-Mo high strength steel having a mixed structure of martensite and bainite

A study has been systematically made of the effect of bainite on the mechanical properties of a commercial Japanese 0.40 pct C-Ni-Cr-Mo high strength steel (AISI 4340 type) having a mixed structure of martensite and bainite. Isothermal transformation of lower bainite at 593 K, which appeared in acicular form and partitioned prior austenite grains, in association with tempered marprovided provided a better combination of strength and fracture ductility, improving true notch tensile strength (TNTS) and fracture appearance transition temperature (FATT) in Charpy impact tests. This occurred regardless of the volume fraction of lower bainite present and/or the tempering conditions employed to create a difference in strength between the two phases. Upper bainite which was isothermally transformed at 673 K appeared as masses that filled prior austenite grains and had a very detrimental effect on the strength and fracture ductility of the steel. Significant damage occurred to TNTS and FATT, irrespective of the volume fraction of upper bainite present and/or the tempering conditions employed when the upper bainite was associated with tempered martensite. However, when the above two types of bainite appeared in the same size, shape, and distribution within tempered martensite approximately equalized to the strength of the bainite, a similar trend or a marked similarity was observed between the tensile properties of the mixed structures and the volume fraction of bainite. From the above results, it is assumed that the mechanical properties of high strength steels having a mixed structure of martensite and bainite are affected more strongly by the size, shape, and distribution of bainite within martensite than by the difference in strength between martensite and bainite or by the type of mixed bainite present. The remarkable effects of the size, shape, and distribution of bainite within martensite on the mechanical properties of the steel are briefly discussed in terms of the modified law of mixtures, metallographic examinations, and the analyses of stress-strain diagrams.

Low temperature improvement of the mechanical properties of 4340 type ultrahigh strength steel with heat treating techniques using interrupted quenching method

In many practical applications the use of ultrahigh strength low alloy steels, such as AISI 4140 and 4340, is limited by their poor ductility and notch toughness in low temperature environments. In recent years, considerable effort has been directed toward improving ambient temperature mechanical properties by various heat treating techniques or compositional modification. ~-~2 Unfortunately, these improvements in mechanical properties have not been reflected in the low temperature mechanical properties of the ultrahigh strength low alloy steels. The basic problem is that the martensitic microstructure tempered to retain a high strength level is inherently susceptible to embrittlement which occurs in low temperature environments. We have given one of the potential solutions to this problem by developing 4140 and 4340 type ultrahigh strength steels having a mixed structure of martensite and lower bainite. 13.14,15 The effectiveness of the mixed structure in improving the mechanical properties is attributed to the fact that lower bainite, which appears in acicular form and partitions prior austenite grains, effectively causes a refinement of the martensitic substructure (i .e. , lath width and packet size) which lead to increased strength and that the lower bainite provides a significant increased resistance to brittle fracture at low temperature environments. As noted, the role of the second phase lower bainite in improving the lower temperature mechanical properties has been found to involve a refinement of the martensitic substructure and increasing resistance to low temperature brittle fracture. Thus, it follows that other ductile second phases should have a similar beneficial effect, provided they appear in acicular form and refine the substructure of the parent martensite, resulting in a higher resistance to cracking in low temperature environments. The obvious alternative second phase to consider is highly tempered martensite. This has the great advantage that the required microstructure can be readily achieved by heat treating techniques using interrupted quenching method. 16'17 Also, there is an added advantage that if the resulting microstructure improves the mechanical properties, the heat treating techniques can be applied to larger sections than the new heat treatment reported previously, ~3'Ja''s because the formation of the microstructure predominantly depends upon temperature.

304L型ステンレス鋼の焼鈍硬化におよぼす加工温度の影響

The relation between deformation temperature and anneal-hardening of type 304L stainless steel was investigated. Specimens were rolled at several temperatures ranging between -60° and 600°C and subsequently annealed up to 900°C.Amount of anneal-hardening was estimated by measuring the change of mechanical properties.The results were summarized as follows:(1) Anneal-hardening occured at 400-500°C before recrystallization when specimens were rolled at a deformation temperature below 600°C and subsequently annealed. Amount of anneal-hardening was increased with falling deformation temperatures. The specimen deformed at 600°C did not show any hardening phenomenon during annealing.(2) Retrogression phenomenon was observed with the specimen anneal-hardened at 400°C after 38% deformation at -60°C. It was presumed that the mechanism of anneal-hardening of type 304L stainless steel was very similar to that of α-brass in consideration of this retrogression phenomenon.(3) Sub-boundaries were formed with the specimens anneal-hardened. It seemed that one of the causes of anneal-hardening in this steel was also attributable to a formation of subboundaries.(4) By measuring magnetic properties of the deformed specimens, it was estimated that Md point of the steel used was about 45°C.(5) In microstructure of the deformed specimens, many parallel stripes were observed in some of grains. These stripes were increased as the degree of deformation was increased and as the deformation temperature was decreased. It was presumed that this structure was a mixed structure of martensite and strain-marking in the case of specimens deformed at a temperature below Md, whereas it was a strain-marking in the case of specimens deformed at a temperature above Md.