Fundamental Concepts of Metals Science Heat treatment of steel: Phase transformations in steel upon heating Pearlitic transformation upon cooling Martensitic transformation upon cooling Heat treatment operations: Annealing Normalization Quenching Tempering

Heat treatment of steel Graphic representation of the heat treatment mode: heating, soaking, cooling The main parameters, defining result of thermal processing, are heating temperature th, soaking time ts and cooling rate vc.

Heat treatment of steel It is accepted to designate the temperatures of transformation, or critical points of steel upon heating, by following manner: the beginning of ferrite – austenite transformation (Fe  Fe) is marked as Ac1 (these points are located on line PSK); the end of ferrite – austenite transformation (Fe  Fe) is marked as Ac3 (these points lie on line GS); the end of the dissolution of cementite in austenite is marked as Accm (the points are on line SE). Points A2 refer to a magnetic transformation, not to a phase one. Attention! Ac3 and Accm points are different for each steel, and Ac1 is the same for all carbon steels: 727 °С. Critical points of steel upon heating

Heat treatment of steel индуктор Heating from a room temperature up to the line PSK does not lead to any changes of pearlitic structure. When temperature Ac1 = 727 °С is reached, transformation of pearlite to austenite begins. Nucleation of a new phase, austenite, occurs in ferrite, on the boundaries between ferrite and cementite (a). The further heating from Ac3 to solidus does not lead to phase transformations (b), but with temperature increase austenite grain grows (c). заготовка Steel structure changes upon heating

Heat treatment of steel Diagram of isothermal austenite transformation for 0.8 % wt. С steel (С-образная диаграмма) индуктор Two curves similar to the letter C show the beginning (curve a–a) and the completion (curve b–b) of austenite transformation to other structures. To the left of the line a–a, the field of overcooled austenite is located. To the right of the line b–b the field of equilibrium products of transformation is situated. заготовка

Heat treatment of steel Nucleation and growth of pearlitic colonies in austenite индуктор Forming crystals of ferrite and cementite have lamellar shape and grow parallel to each other in both sides from grain boundary (b). Pearlitic colonies originate simultaneously in other sections of austenite grain (c). Diffusive transformation develops until the initial phase – austenite – fully disappear (d). It is possible to represent this process as following brief expressions: Austenite0.8  Ferrite0.02 + Cementite6.7, or Austenite0.8  Pearlite0.8. заготовка

Heat treatment of steel Products of diffusion transformation of austenite Structure Temperature of formation, С Interlamellar distance, µm Hardness, HB Coarse pearlite  670 0,5–0,7 170–230 Pearlite (sorbite) 590–640 0,3–0,4 230–330 Fine pearlite (troostite) 550–580 0,1–0,2 330–400 индуктор Ferrite Ferrite заготовка b а c а b c Ferrite Cementite Microstrutures and schemes of structures: а is pearlite, b is sorbite, c is troostite

Heat treatment of steel индуктор a b Scheme of austenite (а) transformation into martensite (b) During the transformation, the iron atoms are displaced to distances less than interatomic ones; carbon diffusion does not happen. In figure (a) two elementary cells of austenite are shown (circles are iron atoms). The carbon atoms remain as interstitial impurities in iron. They occupy octahedral voids in the midpoints of the edges and in the centers of the cubs, which are marked with crosses. In the FCC lattice of austenite, one can mentally select a tetragonal lattice of martensite (in the figure it is shown in bold lines). The austenite lattice rearrangement occurs according to crystallographic planes, which are close in structure to the specific planes of the martensite lattice. заготовка

Heat treatment of steel индуктор Unit cell of martensite Martensite plate photography заготовка Scheme of martensite formation

Heat treatment of steel Dependence of the hardness of martensite on the carbon content Carbon content in martensite, % 0,2 0,3 0,4 0,5 0,6 0,8 1,0 Hardness, НRC 40 48 55 59 62 65 66 индуктор заготовка Dependence of temperatures of the beginning and the termination of martensitic transformation on the carbon content in steel

Heat treatment of steel Martensite microstructure индуктор заготовка a b Needle martensite in high carbon steel (a) and lath martensite in medium carbon steel (b)

Heat treatment operations Annealing Full annealing conditions Partial annealing conditions индуктор Full annealing of hypoeutectoid (structural) steels is performed with the aim of complete phase recrystallization. The steel is heated 30–50 above the Ас3 critical point (GS line) and slowly cooled after a short soak. Virtually parts are cooled inside the furnace; cooling rate is 30–100 С/h. Partial annealing of hypereutectoid (tool) steels is performed with the aim of obtaining the structure of granular perlite. To do this, the steel is heated 30–50  above the Ac1 critical point (PSK line). There are several different annealing modes. заготовка

Heat treatment operations Annealing and normalization индуктор Temperature ranges of steel heating for annealing Diffusion annealing of steel cast and forged pieces is performed in order to eliminate the heterogeneity of the cast or deformed structure. Elimination of microsegregation is achieved through diffusion processes. Normalization consists in heating the steel to temperatures 50–70 ° above the GSE line and in air cooling after a short exposure. Therefore, normalization results in a finer grain structure than the full annealing: fine-lamellar perlite or sorbite. заготовка

Heat treatment operations Quenching индуктор Temperature ranges of steel heating for quenching Quenching consists in heating steel above the phase transformation temperature followed by sufficiently rapid cooling (at a rate greater than critical). The purpose of quenching is to obtain a non-equilibrium structure, that is martensite, a supersaturated solid solution of carbon in -iron. The practical goal is to obtain maximum hardness for this steel grade. заготовка

Heat treatment operations Quenching индуктор Quenching variations by cooling method : 1) Continuous hardening (quenching in the same medium) (curve 1). 2) Interrupted quenching (quenching in two mediums) (curve 2). 3) Step quenching (curve 3), in which the heated part is immersed in a liquid medium with a temperature of 20–30 above the point Ms. 4) Isothermal hardening (curve 4). It differs significantly from other methods. Here the part is in a cooling medium at a temperature of bainitic transformation until complete decomposition of austenite occurs. In all the previous cases, the martensitic structure forms during quenching, and during isothermal quenching bainite is formed. заготовка

Heat treatment operations Quenching Quenching is carried out by rapid cooling (vcool > vcr) in quenching liquids (water, water solutions, mineral oils)

Heat treatment operations Quenching a b Hardenability is an ability to receive high hardness upon quenching. Hardenability depends on the carbon content in steel and is characterised by the maximum possible hardness (HRC) for the given steel grade. Hardness penetration is steel ability to receive the hardened layer of certain depth. Rate of cooling decreases from the surface of the part to the center, therefore it can happen that the cooling rate will be lower than critical one in the sample’s core for the thick enough samples (figure b). In this case only surface layer will receive hardening and acquire the martensite structure whereas the core will stay not-quechned and composed of soft ferrite and pearlite structures. The characteristic of hardness penetration is a value of critical diameter. Critical diameter is the maximum diameter of cylindrical bar which is quenched through in the particular cooling medium.

Heat treatment operations Quenching Quenching of solid armor hull in oil

Heat treatment operations Tempering Tempering is carried out by heating the steel up to temperatures below critical ones with the subsequent cooling usually in air. The purpose of tempering is to form a complex of service properties required, obtain more stable steel structure than as-quenched one, and reduce internal stresses. Tempering is the last operation in a technological process of steel heat treatment, therefore the structure formed by tempering should provide the necessary properties for service. индуктор заготовка As a result of tempering, when martensite is heated, carbon is precipitated from the solid solution. Equilibrium phases are formed: ferrite and cementite. This transformation can be written in short as follows: M0,8  F0,02 + C6,69. But the decomposition of martensite produces spheroid structures, in contrast to lamellar ones, as when cooling austenite.

Heat treatment operations Tempering индуктор Microstructure of tempered steel (secondary sorbite) Dependence of steel mechanical characteristics on tempering temperature Depending on the heating temperature, the tempering is subdivided into 3 types: low tempering (150–200 С), medium tempering (300–450 С) and high tempering (500–650 С). With an increase in temperature and duration of tempering, the ductility and impact strength of steel increase, but its hardness and strength decrease. заготовка