Malleable Steel Features

The grades in the Chinese national standard (GB9440-88) basically conform to the international standard (ISO5922-1981).

Graphitization annealing mainly involves the mechanism of solid state graphitization, the influence of graphitization annealing process and the influence of various elements on solid state graphitization.

(1) Solid state graphitization mechanism. The cementite in the white mouth green body is an unstable phase, and as long as the conditions are met, it can be decomposed into stable phases-ferrite and graphite, which is the solid-state graphitization process. The necessary condition is that whether solid-state graphitization of white cast iron can proceed depends on both the thermodynamic and kinetic conditions of cementite decomposition and graphite growth. From a thermodynamic point of view, cementite can also undergo solid-state graphitization when it is kept at a temperature lower than that of the iron-carbon phase diagram A. However, whether the decomposition of cementite can be carried out continuously and whether the graphitization process can be finally completed depends to a large extent on the diffusion ability and possibility of carbon atoms after the decomposition of cementite, so that the old phase disappears and the new phases are formed. Dynamic conditions such as resistance factors. In the presence of multiple phases of cementite and matrix, graphite nuclei are most likely to be generated at the interface between cementite and surrounding solid solution; if there are various inclusion particles such as sulfides and oxides in cast iron, graphite nuclei is easier to form. In order for the graphite nuclei in white cast iron to continue to grow, the conditions for strong diffusion of carbon atoms must be met. Pure iron-carbon alloys are more difficult to graphitize, and when there are elements that promote graphitization, the graphitization process can be accelerated. Many viewpoints on the mechanism of solid-state graphitization of cast iron are proposed based on the traditional two-stage annealing process. At the high temperature stage, when heated to the austenite temperature region, it goes through four steps: nucleation at the austenite-cementite interface; cementite dissolves in the surrounding austenite; carbon atoms in the austenite Diffusion from the austenite cementite interface to the austenite-graphite interface; the precipitation of carbon atoms on the graphite core leads to the growth of graphite. During this stage of annealing, . The cementite is continuously dissolved, and the graphite continues to grow until the cementite is completely dissolved. At this time, the equilibrium structure of cast iron is austenite and graphite. In the low temperature stage, the eutectoid transformation into ferrite occurs, and finally the equilibrium structure of ferrite and graphite is formed. Due to the advent of the low-temperature graphitization annealing process, the solid-state graphitization mechanism has developed accordingly. The heating temperature is not higher than A, temperature, and only in the heat preservation stage of 720 ~ 750 ℃, the cast iron structure is directly transformed from the original pearlite and ledite to ferrite and graphite. The key is to improve the graphitization kinetic conditions at lower temperatures and to enhance the inherent graphitization factor of cast iron. Such as refining cementite, refining grains to increase the interface, increase the dislocation density, thereby increasing the number of initial graphite cores to reduce the diffusion distance.

(2) The influence of graphitization annealing process. In the first stage, the temperature is usually 920-980 °C for heat preservation, the eutectic cementite in the ledeburite continues to dissolve into the austenite and gradually disappears, and the flocculent stone zero gradually forms. In the second stage, the commonly used temperature is 710-730°C for heat preservation, or the temperature is slowly lowered from 750°C (3-5°C/h) to 700°C. The common temperature of pretreatment is divided into high temperature pretreatment, which is kept at about 750 °C for 1 to 2 hours, and low temperature pretreatment, which is kept at 350 to 450 °C for 3 to 5 hours. Its function is to increase the number of stone particles, reduce the diffusion distance of carbon atoms, shorten the annealing cycle, and improve the shape of graphite.

(3) The influence of various elements on solid-state graphitization. Carbon can promote stone emeraldization, increase the number of annealed graphite cores, and shorten the stone optimization time, especially the second-stage graphitization time. Silicon strongly promotes graphitization and can promote the decomposition of cementite. Therefore, increasing the silicon content in the molten iron within the allowable limit can effectively shorten the annealing time of the first and second stages. Adding ferrosilicon or silicon-containing composite inoculants before the furnace can cause large concentration fluctuations, which is conducive to low-temperature graphitization. Manganese can form MnS with sulfur, so the graphitization time can be shortened within an appropriate content range. However, when the amount of free manganese (manganese combined with sulfur to form excess manganese other than MnS) exceeds a fixed value (>0.15% to 0.25%) or is insufficient (negative value), it will hinder graphitization, especially the second stage graphite. change. Sulfur strongly hinders graphitization. When the sulfur content is not very high (<0.25%), its harmful effects can be neutralized by manganese. When the sulfur content is high, it makes graphitization annealing difficult. Phosphorus weakly promotes graphitization during solidification and has little effect on solid-state graphitization during annealing. When it exceeds a certain amount, it will slightly hinder the second-stage graphitization. Others such as chromium, molybdenum, vanadium, tellurium, etc. have a strong hindering effect on graphitization; aluminum, zirconium, and calcium have a strong promoting effect on graphitization. [1]