How to improve the hardness of gray iron

improve the hardness, strength and cutting performance of gray iron?

1. Burden ratio Burden ratio uses the method of pig iron + scrap steel + returning furnace material + carburant to improve the hardness of gray iron castings by using nitrogen in the carburant to change the shape and length of graphite.

2, control of chemical composition (1) Many smelting companies believe that sulfur is harmful, the lower the content of sulfur in molten iron, the better, in fact, this is not the case, in gray iron castings should consider "silicon carbide" and "manganese sulfur ratio". Mn = 1.71S +(0.2-0.5).

HT250 chemical composition table: CPSiMnTiS3 ~ 3.3 ≤ 0.121.65~2.050.7~1.1 ≤ 0.05 ≤ 0.12(2) low alloying, add one or two alloy elements, when adding, should consider the content of carbon element, do not blindly pursue hardness.

3. For gray iron castings, increasing the temperature of molten iron within a certain range can refine graphite, compact matrix structure, and improve the tensile strength and Brinell hardness of cast iron. It is better to control the overheating temperature of molten iron at 1500-1530 ℃ and the overheating time within 10min.

4. Inoculant and Inoculation Method The inoculation treatment of gray cast iron is usually to slowly add inoculant (the most commonly used 75Si-Fe) to the molten iron after being discharged from the furnace. The hardness of gray cast iron after inoculation treatment will tend to be uniform, which improves the mechanical processing performance and increases the strength of gray cast iron. The hardness of gray iron castings is standard when HB170-240.

2. how to increase the strength of gray cast iron materials by niobium

niobium's strong affinity for carbon complicates its recovery in cast iron and similar high-carbon molten baths. A layer of niobium carbide is rapidly formed at the ferroalloy or melting interface, and its dissolution determines the recovery rate of niobium in the molten bath. The behavior of niobium in cast iron is further determined by the study of the dissolution process of niobium iron.

According to the brake disc performance and casting process requirements, the experimental niobium iron purity of 65% of the standard niobium iron, niobium iron melting point range of 1580 to 1630 degrees C (solidus and liquidus temperature), much higher than cast iron, slightly higher than cast steel. Niobium does not exothermically react with iron. Therefore, ferro-niobium in molten iron is not a melting process, but a dissolution process based on interfacial diffusion. This dissolution process requires a certain period of time. According to the experimental conditions, the niobium iron block is processed into a cylindrical shape with a Ф size of 5mm × 30mm. The experimental equipment includes a 10kg medium frequency induction furnace with a melting volume of 7kg for each test, Ф a 35mm × 150mm sand casting mold. Before the test, the sand mold is preheated to 200 ℃, and the molten iron is preheated to 1500 ℃ for pouring. Analytical instruments include 4XB metallographic microscope, scanning electron microscope, MCO120-MHV-2000 microhardness tester, etc.

finally come to the conclusion

1) ferro-niobium is not a melting process in molten iron, but a dissolution process based on interfacial diffusion. On the diffusion front, niobium interacts with carbon in graphite, which makes the graphite morphology become fine and curled, and in the direction away from the diffusion, the graphite morphology is not affected much due to the low niobium content.

2) In the vertical direction, since the diffusion temperature is higher than that in the horizontal direction, the width of the diffusion layer is also larger, that is, the vertical direction is more conducive to the dissolution and diffusion of the niobium iron.

3) Studies have shown that the effect of niobium on the pearlite matrix is to refine it, thereby increasing the strength of the material.

In general, the higher the hardness and strength of cast iron, the lower the metal cutting performance, and the lower the life expectancy of the blade and tool. Most types of cast iron used in metal cutting production have generally good metal cutting properties. The metal cutting performance is related to the structure, and the harder pearlitic cast iron is also difficult to process. Flake graphite cast iron and malleable cast iron have excellent cutting properties, while ductile iron is quite bad. The main types of wear encountered when machining cast iron are: abrasion, bonding and diffusion wear. Abrasion is mainly produced by carbides, sand particles and hard cast skin. Bond wear with chip deposits occurs at low cutting temperatures and cutting speeds. The ferritic portion of cast iron is most easily welded to the insert, but this can be overcome by increasing the cutting speed and temperature.

on the other hand, diffusion wear is temperature dependent and occurs at high cutting speeds, especially when high strength cast iron grades are used. These grades have a high resistance to deformation, resulting in high temperatures. This wear is related to the interaction between the cast iron and the tool, which makes some cast iron need to be machined at high speed with ceramic or cubic boron nitride (CBN) tools to obtain good tool life and surface quality. The typical tool properties required for the processing of cast iron are: high thermal hardness and chemical stability, but also related to the process, workpiece and cutting conditions; the cutting edge is required to have toughness, heat fatigue wear and edge strength. The satisfaction of cutting cast iron depends on how the wear of the cutting edge develops: rapid dullness means that hot cracks and notches are generated, and the cutting edge is prematurely broken, the workpiece is damaged, the surface quality is poor, and the waviness is too large. Normal flank wear, balance and sharp cutting edges are what generally need to be worked hard to achieve.

For how to improve the strength of gray cast iron, gray cast iron researchers at home and abroad have carried out a lot of research work, summed up in the following ways:

1. Optimizing Carbon Equivalent CE and Si/C Ratio

, due to the extremely low strength and hardness of graphite, it can be regarded as zero relative to iron, and the serious fragmentation effect of flake graphite on the matrix, the higher the carbon content in gray cast iron, in general, the lower the strength and hardness, that is, the tensile strength of gray cast iron decreases with the increase of carbon equivalent.

In the course of the development of high-strength gray cast iron, it has been an important measure to reduce carbon equivalent and increase manganese content, thereby increasing the proportion of pearlite in gray cast iron and improving the tensile strength of gray cast iron. However, the method of improving the tensile strength of gray cast iron by reducing the carbon equivalent also brings many adverse effects, such as poor casting process performance. White mouth tends to increase and is difficult to process. Large stress, easy to crack; Large shrinkage of molten iron, easy to produce shrinkage porosity, resulting in leakage; High sensitivity of casting section, easy to produce waste, etc. Therefore, it has not been widely used.

It is generally believed that under the same carbon equivalent conditions, the tensile strength can be increased by 30~60MPa with the increase of Si/C? ratio. This is because, under the condition of the same carbon equivalent, with the increase of silicon carbon ratio, the number of austenite dendrites of gray cast iron increases.

2. Optimization of Mn and S content and Mn/S ratio

(1) Effect of manganese on tensile strength of gray cast iron

manganese is an element that expands the austenite region, increasing the manganese content in molten iron can effectively reduce the austenite eutectoid transformation temperature, which is beneficial to the formation of pearlite and increase the number of austenite dendrites, and the transformation from austenite to pearlite is carried out at a lower temperature, thus promoting the refinement of pearlite flakes and reducing the spacing between pearlite flakes. Manganese is a strong carbide-forming and stable carbide element, and manganese displaces iron in? Fe3C?, forming (Fe,?Mn) 3C? constituting stronger and harder pearlite, promoting the formation of pearlite. Manganese can be infinite solid solution in austenite, but also solid solution in the matrix tissue, strengthen the matrix, improve the strength of gray cast iron.

(2) Effect of sulfur on tensile strength of gray cast iron

sulfur plays a dual role in gray cast iron. It is generally believed that sulfur is a strong stable carburizing body in the breeding cast iron, hindering the carbonization of stone elements, but from the thermodynamic analysis, sulfur can reduce the solubility of carbon in iron liquid, and manganese, rare earth to form MnS and ReS will become graphite non-spontaneous.

the core of nucleation, it can promote graphitization, so sulfur is an indispensable element to promote the incubation reaction.

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