Commercial Carbon Steel Research Articles

Graphitization in High Carbon Commercial Steels

Graphitization kinetics in two commercial high carbon steels, AISI types 1075 and 1095, are studied by conducting a series of isothermal annealing treatments in the temperature range of 560 to 680° C for periods of time ranging from 20 to 500 h. The samples selected were collected along the processing route in a commercial production line dedicated to the fabrication of thin strip. The structures studied were those of hot rolling (consisting of fine pearlite), cold rolling (spheroidized carbides within a deformed ferritic matrix), and subcritical annealing (spheroidized carbides in undeformed ferrite). The samples obtained from hot rolled coils do not graphitize, whereas those cold rolled graphitize at a rate that depends on the type of steel and degree of deformation. No graphite was found in samples from the lower carbon steel, which were subcritically annealed, although they were observed in specimens from the other steel, which were cold rolled to a reduction of 50% prior to the subcritical annealing.

Precipitation at Transformation Interfaces

AbstractAlloy carbide precipitation associated with the transformation interface between austenite and ferrite, and austenite and pearlite in steels can result in a fine particle dispersion that can contribute significantly to strength in a wide range of commercial carbon steels. Studies of this precipitation reaction are hindered by the fact that the transformation interface is lost upon cooling to room temperature, either by further transformation or by decomposition of the residual austenite phase to martensite. In the present work this has been avoided by developing an alloy in which the precipitation reaction occurs, but in which the austenite is stabilised to room temperature, thus allowing a detailed examination by TEM and APFIM techniques of the interfacial region.

Intergranular microchemistry and stress corrosion cracking

An important contributory role of grain boundary segregation of residual impurities in the intergranular stress-corrosion cracking of carbon and low alloy steels is proposed. Experimental results are presented of the stress corrosion susceptibility of mild steel in nitrate solution, and in relation to varying grain boundary composition as monitored by Auger electron spectroscopy. The harmfulness of a particular impurity element depends on three factors: its bulk level; its segregation thermodynamics and kinetics resulting in an equilibrium enrichment at the grain boundaries; and its ability to promote electrochemical dissolution of the grain boundary. A hierarchy of impurity elements that exacerbate stress corrosion cracking is presented and correlated with equilibrium oxidation potentials. The results and simple model allow the prediction of the relative harmfulness of impurity elements with respect to intergranular stress corrosion in commercial carbon and low alloy steels from a knowledge of the bulk concentration only. This enables significant improvements in performance to be designed in the alloy by respecifying lower levels of only the one or two highly detrimental impurities.

炭素鋼と溶融亜鉛との反応

In order to study the effect of alloying elements in steel on the Fe-Zn reaction, commercial carbon steels were immersed into pure molten zinc for 60∼6000 s at 713∼873 K. The alloy layers formed on the steel surface were observed and the quantities of iron reacted with zinc were measured. The results are as follows:(1) At 713 K, the alloy layers consisted of Γ, δ1, (δ1+η) and ζ. The ζ was the aggregate of fragmented ζ crystals, and this phase dropped off into η for immersion longer than 3000 s. Up to 763 K, ζ was also formed, but it grew to a large columnar crystal. Above 773 K the alloy layer sequence was similar to that of pure iron samples.(2) Although Fe3ZnC was formed, the sequence of the alloy layer formed on the pearlite was identical to that formed on the ferrite.(3) FeSi was formed in the alloy layer, but it could not be found under the optical microscope.(4) The δ1 thickness-temperature curves had two peaks at 733 and 813 K. The fact that the surface of δ1 was covered entirely by ζ caused the peak at 733 K. The peak at 813 K was due to δ1K formation and a drop off into η.(5) The temperature dependence of iron loss corresponded to that of iron quantity remained in the alloy layer up to 763 K, and to that of iron quantity dissolved into the zinc bath above 813 K.

A study of particle arrangement in magnetic tapes

Particle dispersion and particle orientation in magnetic tapes are experimentally interpreted by using a large-scale model which consists of commercial carbon steel wires and a vinyl chloride binder. Results suggest that the particle orientation in the plane through the thickness of the coating was broader than that in the plane of coating. Even though the micromagnetic equations are not properly scaled, comparison of the large-scale model and γ-Fe 2 O 3 assemblies shows general agreement with respect to packing fraction. The coercive force of an isolated single γ-Fe 2 O 3 particle is found to be 420-440 Oe, compared to 380 Oe found for packing fractions used in commercial magnetic tapes.

Kinetics of the massive martensitic transformation in commercial carbon steels

1. Raising the quenching temperature of commercial carbon steels to 1180–1200°C produces a mixture of massive and twin martensite. 2. The temperature range in which massive martensite is formed is higher than the range in which twin martensite is formed and coincides with the range in which bainite is formed. 3. The limit concentration of carbon corresponding to a shift from the massive to twin martensite transformation depends on the cooling rate-with increasing cooling rates the carbon concentration at which twin martensite is formed decreases. 4. The critical cooling rate for formation of massive martensite is lower than for twin martensite. 5. The nature and kinetics of the formation of bainite and massive martensite are similar. Differences in the structures are due to the carbon content, since diffusional redistribution of carbon occurs in the process of the transformation. Thus, it is expedient to use the term bainite for carbon-containing alloys and massive martensite (isothermal) for carbon-free alloys (<0.1% C).

液体アンモニア中における鋼の腐食割れ感受性について

Stress corrosion cracking (SCC) tests of commercial carbon steels were conducted both in a liquid ammonia saturated with NH4CO2NH2 at 30°C for 500 h and in a field liquid ammonia. H2S cracking tests for the same specimens were also carried out. The results obtained are as follows:(1) In the liquid NH3-NH4CO2NH2 SCC of quenched steels occurs if the hardness of the specimens is high, and the increase of carbon content accelerated SCC. This result is similar to that in H2S cracking tests.(2) In a liquid NH3 SCC of quenched steels tends to occur if the content of alloying elements in the steels is high, even though their hardness is low. Nickel in the steels has the most remarkable effect on SCC and many corrosion cracks in the liquid NH3 were observed on the steels containing Ni.(3) Tempering above 300°C after quenching decreases markedly the susceptibility to SCC of all the types of steels in the liquid NH3-NH4CO2NH2.(4) The hydrogen permeability into the steel is increased by the addition of NH4CO2NH2 to liquid NH3, and the susceptibility to SCC of each steel in the liquid NH3-NH4CO2NH2 is similar to that to H2S cracking. Therefore, the propagation of cracks in the steel seems to be due partly to hydrogen emblitlement in the liquid NH3.

Scatter of Fatigue Strength in Medium Carbon Steels

A series of fatigue tests were conducted, from a statistical viewpoint to obtain foundamental data on the scattering characteristics of fatigue strength, on 3 kinds of commercial plain carbon steels of 0.45% C type and 2 kinds of 0.25% C type. The analysis of the results revealed that the scatter in fatigue strength of smooth specimens may generally be fitted to the normal distribution, and it is possible to obtain P-S-N curves by shifting laterally on S-N diagram the plot of median fatigue strengths estimated by Probit analysis from the data obtained at different numbers of cycles.The scatter in fatigue strength of smooth specimens made of the same steel subjected to a constant condition is affected by the difference in the sample rods from which the specimens were made. For the specimens subjected to different heat treatment conditions, the scatter tends to increase as the fatigue strength becomes higher. The variation coefficient of fatigue strength at 107 cycles is estimated to be approximately 2.5% for all the cases studied, except for the case of 0.45% C steel tempered at 350°C to achieve high brittleness, where it is 4.9%.For notched specimens of the same steel, on the other hand, the variation coefficients are 6.3% and 5.3% for the specimens tempered at 600°C and 350°C, respectively. From the fact that the notch causes a large increase in the variation coefficient for ductile materials but not so much for brittle materials, the process of crack propagation is thought to have great influence on the scattering phenomenon of fatigue strength.

Grain refinement of steel by cyclic rapid heating

A recently developed technique of grain refinement by rapid cyclic heat treatment has been applied to a commercial medium carbon steel. The changes in the microstructure, microfracture and tensile behavior as related to the reduced grain size are reported. An ultimate refined grain size of about 4 μ is achieved after four cycles. The nature of dimple fracture as revealed by microfractography indicates that the increased yield stress is associated with improved ductile fracture behavior.

共析炭素鋼の上部ベイナイトおよび恒温パーライト変態におよぼすオ一ステナイト化処理条件の影響

As has previously been reported on the lower bainite transformation, the effects of austenitizing temperature and time (longer than 60 min) on the upper bainite transformation (at 380°C) and the isothermal pearlite transformation (at 675°C) are investigated for a commercial eutectoid carbon steel heat treated at 1200°C for 20 min like those on the lower bainite transformation which were reported earlier. The principal findings are as follows.(1) The amount of bainite formed during a certain transformation period increases to a maximum with increasing time and then starts to decrease. On the other hand, the amount of pearlite formed decreases to a minimum as the time increases, after which it increases to a maximum and again decreases.(2) The higher the austenitizing temperature the shorter is the time to reach the maximum, and the lower the temperature the higher is the maximum in both bainite and pearite transformations. Besides, the higher the temperature the shorter is the time to reach the minimum pearlite formation, and the lower the temperature the higher is the minimum.(3) Bainite or pearlite starts to nucleate at grain boundaries in the case of a short austenitizing time. However, this tendency of nucleation becomes a little weaker as the time increases and the nucleation also starts inside the grains. Furthermore, in the case of a long austenitizing time, bainite plates and pearlite nodules at grain boundaries or inside the grains are fine compared to those at grain boundaries in the case of a short austenitizing time.(4) Nucleation of bainite or pearlite inside the grains is caused by the nuclear action by AlN. The nuclear action by AlN seems to be stronger in the bainite tranformation than that in the isothermal pearlite transformation.(5) The value of activation energy obtained by assuming that a process to show the maximum amount of transformation is associated with that for thermal activation can be taken as 33.8 kcal/mol in all the upper bainite, lower bainite and pearlite transformations.

パーライトの成長に関する一考察

パーライトの成長に関する一考察

炭素鋼のセメンタイト球状化におよぼす加工の影響

In order to shorten the annealing time of some hardenable steels, the anneal treatments that comprise proper working in the annealing process were studied. Firstly, the effects of plastic deformation at various temperature on the spheroidization of three kinds of commercial carbon steels (0.44, 0.82 and 1.23%C) were investigated by microscopic observation and hardness measurement. Results obtained were as follows:(1) All the steels, deformed above the 13% working ratio at the highest heating temperature and then thermal cyclic treated, showed a slightly lower constant hardness and a better spheroidized structure than those without working.(2) The steels, 13% deformed over the metastable austenitic temperature range and then thermal cyclic treated, were not affected remarkably by the deforming temperature. Only the 0.82%C steel showed a slightly lower hardness by working at 600°C. The hardness of all the steels after this treatment were comparatively higher than that of the steels worked at the highest heating temperature.(3) All the steels, 13% deformed at room temperature and at 500°∼700°C without prior-heating at the highest heating temperature and then thermal cyclic treated, showed a lower hardness and a mixed spheroidal structure of coarse and fine cementites by working at 600°C.Particularly, the 0.44%C steel showed a better spheroidized structure than that obtained by any other methods.