Ferenc Tranta

In Situ Optical Microscope Examinations of the ε↔γ Transformations in FeMn(Cr) Austenitic Steels during Thermal Cycling

A group of austenitic steels exhibit high deformability and strength due to TRansformation Induced Plasticity (TRIP) and/or TWinning Induced Plasticity (TWIP). The phase transformations of the TRIP and TWIP steels have been examined in details in many FeMnX alloy systems (X: Ni, Al, Si). However, less attention was given to the FeMn(Cr) alloys. The γ ↔ ε transformations in the austenitic FeMn(Cr) alloys have been examined during heat cycling by in situ optical microscopy and DSC measurements.

Deforming Texture Effect on Phase Transformation in Stainless Steels

In stainless steels during the non-equilibrium transformation of aus tenite the deformed bcc α’ martensite and hexagonal, non-magnetic ε martensite also can form. These transformations can be induced by cooling (under critical temperature) and/or mechani cal deformation. The appearance of ε martensite has an effect on the deformation ability and other prope rties. The experimental results associate the ε martensite formation with the plastic deformation behaviour (TRIP effect), while the transformation of ε martensite into α’ martensite is accompanied with hardening. The present phases hardly determine the cold rolling properties. During the deforming of α’, ε, γ phases in stainless steels the texture also developes. The i nitial strong deforming γ texture changes with the appearance of α’ martensite. We suppose that the transformation takes place at certain places in an anisotropic wa y due to the deforming texture. The texture of α’ martensite develops by the initial anisotropy of parent phase a nd/or the phase deformation. Introduction Martensite has long been used to designate the hard microstructur e found in quenched carbon steel. If the steel with the carbon content higher than 0.6wt% is heated above the austeniti c range and than quenched at a sufficiently high cooling rate the fcc austenite tra nsforms into the metastable body centered tetragonal phase ( ). Perhaps the most important aspect in martensitic transformati n involves a special crystallographic relationship between the marte nsite and austenite, which allows a fast growth mechanism. The crystallography is very simil ar to the crystallography involved in deformation twinning. Thus in general when a martensitic transform ation is coupled with an externally applied stress other phenomena such as stress or strai n induced martensitic transformation, the generation of new nucleation sites, and transformat ion induced plasticity (TRIP) may occur. The most commonly used austenitic steel has a tendenc y to transform into α’ martensite and hexagonal ε martensite during thermomechanical treatment. The ε martensite has a well defined crystallographic relationship to the parent austenite phase, it forms close to the stacking faults with fine lathes on the {111} plate of austenite. The thickness of the ε lath is 300 nm or less. [1] Between Ms and Md temperatures the formation of martensite can be induced by ela stic and/or plastic deformation. Within this temperature intervals the driving force for the reaction consists of 1) the free energy difference between the martensitic and aust enitic states and 2) the externally applied stress. The authors propose to distinguish between the stress-i nduced and strain-induced formation of martensite. Martensite is considered to be stress induced when it forms as a result of elastic stresses from an external load (below the actual yi eld strength of austenite). Martensite is strain induced when the slip in the austenite precedes its format i n. (σA-M>σyield A, the lowest temperature is Ms σ ). The austenitic stainless steels have a wide range of usage i n th chemical industry and also in the processing of household goods. The most commonly used processing for the se goods is the cold working. So during the deformation of metastable austenitic CrNi st eels he and ε martensite can form. According to the chemical composition of steel and the condition of def rmation different transformations take place such as γ→ε, ε→α’ , γ→α’ , γ→ε→α‘. The deformation induced Materials Science Forum Online: 2003-01-15 ISSN: 1662-9752, Vols. 414-415, pp 281-288 doi:10.4028/www.scientific.net/MSF.414-415.281

Characteristics of Martensitic Transformations Induced by Uni-Axial Tensile Tests in a FeMnCr Steel

Austenitic FeMnCr steels have high strength, high toughness and formability because of the stress-and strain-induced γ→α and γ→ε martensitic phase transformations. These are the so-called TRIP (Transformation Induced Plasticity) and TWIP (Twining induced Plasticity) effects. TWIP steels deform by both glide of individual dislocations and mechanical twinning [1]. The type and mechanism of the austenite→martensite transformation depends on the composition, deformation rate and temperature. The ratio and quantity of the resulting phases determine the properties of the product. It is known that austenitic steels can transform into α and/or ε martensite phases during plastic deformation The characteristics of the martensitic transformations induced by uni-axial tensile tests between room temperature and 200°C in a FeMnCr steel with 2,26 w% Cr content were examined. Mechanical properties as, yield stress were determined from tensile tests. Metallographic examinations, quantitative and qualitative phase analysis by X-ray diffraction were carried out on the uniformly elongated part of the samples (cross, longitudinal sections).

Investigation of thermomechanical treated austenitic stainless steel

During thermomechanical treatment of austenitic stainless steel a’ martensite and e martensite form in the austenite matrix. The martensitic transformation and deforming existing together result a high elongation at the investigated steel belonging to the TRIP grades. The amount of a’and e martensite depends on the strain level as well as on the deforming temperature in this steel. In the course of thermomechanical treatments we measured the amount and texture of the existing phases at different temperature and strain. It has been stated that the martensites are dominant in low temperature range, they have a considerable amount, and the transformation from e martensite to a’ martensite also takes place. The amount of a’ martensite increases by increasing the strain while the amount of e martensite shows a maximum. By investigating the relationship between the quantity of existing phases and the effect of texture, it has been stated that the transformation takes place in certain given plates in the textured matrix.

A New Method to Examine the Annealing of Two Alloying Elements of 3004-Grade Cast Aluminum by Resistivity and Thermoelectric Power

Simultaneous measurement of electric resistance and thermopower the concentration of manganese and magnesium in the solid solution were determined by using the Matthiessen and Nordheim-Gorter relationships, in case of homogenized continuously cast AlMn1Mg3 alloy. It can be stated that the quantity of magnesium changes only in a small extent in the course of heat treatment while the manganese precipitates from the oversaturated structure between 400°C and 600°C then it dissolves again over 600°C.

The Effects of Heat Treatment and Deformation on the Martensitic Transformation of FeMn (Cr) Steels

The ε martensite phase was investigated in three types of FeMn (Cr) steels by means of metallographic, EBSD and XRD methods as well as by dilatometric and DSC examinations during heating and cooling. In the course of the latter examinations, the transformation temperatures (Ms, As) of ε martensite were determined in the previously deformed samples as well. The results were also compared to the thermodynamic data published in the references describing the γε and γα transformations.

Solid-State Transformation Produced by Laser Treatment and Mechanical Alloying of Fe-Ni-Cu(P) Powders

The comparison of the phase transformations going on due to high energy ball milling (HEBM) and produced by pressure-less Direct Metal Laser Sintering (DMLS developed by EOS company) was carried out, by using an α-Fe, Ni and Cu3P powder mixture. It could be shown by X-ray diffractograms (XRD) of the two type of products, that by mechanical alloying a similar phase transformation occurs due to solid state reactions between the metal partners as in the case of laser sintering, in a given range of laser scanning speed in a laboratory laser equipment. According to the XRD evaluation the same metastable, γ-steel like phases were formed.