What Are Malleable Fittings?

Malleable cast iron, which is cast from a certain chemical composition of molten iron into a white blank, and then annealed, has high strength, plasticity and impact toughness, and can partially replace carbon steel.

Chinese name: Malleable cast iron
Foreign name: malleable cast iron

Meaning: Ductile cast iron made by heat treatment
Alias: Ma Tie, Ma Steel

Basic information of malleable cast iron

English name: malleable cast iron

Pronunciation: k duàn zhù ti

Malleable cast iron: ductile cast iron made from white cast iron after heat treatment.

Alias: Ma Tie, Ma Steel,

Introduction to malleable cast iron

A high-strength and toughness cast iron obtained from white malleable cast iron through graphitizing annealing treatment. It has high strength, plasticity and impact toughness, and can partially replace carbon steel. Compared with gray cast iron, malleable cast iron has better strength and plasticity, especially better low temperature impact performance, wear resistance and vibration resistance is better than ordinary carbon steel. Because this type of cast iron has a certain degree of plasticity and toughness, it is commonly known as martensitic steel, martensite, also known as ductile cast iron or ductile cast iron. Black heart malleable cast iron is used for parts subject to shock or vibration and torsional loads, and is often used in the manufacture of automobile rear axles, spring brackets, low-pressure valves, pipe joints, tool wrenches, etc. Pearlitic malleable cast iron is commonly used to make wear-resistant parts for power machinery and agricultural machinery. There are international examples of automotive camshafts. White-heart malleable cast iron is rarely used due to its long malleable annealing time (see Ferritic malleable iron, Pearlite malleable iron, and White-heart malleable iron).

Malleable cast iron production process

First cast into white cast iron parts, and then through malleable annealing (the malleable annealing decomposes cementite into floc graphite) to obtain malleable cast iron parts.

chemical composition:

The chemical composition of malleable cast iron is: wC = 2.2% 2.8%, wSi = 1.0% 1.8%, wMn = 0.3% 0.8%, wS0.2%, wP0.1%.

There are two types of structure of malleable cast iron:

Ferrite (F) + flocculent graphite (G);

Pearlite (P) + flocculent graphite (G).

Malleable cast iron use

Malleable cast iron Automotive metal materials. The white mouth blank is cast from a certain chemical composition of molten iron, and after graphitizing annealing, graphite is mainly flocculent, flocculent, and sometimes a small amount of cast iron. Compared with gray cast iron, malleable cast iron has higher strength, toughness and impact toughness. The malleable cast iron is divided into four types: black heart malleable cast iron, pearlite malleable cast iron, white heart malleable cast iron and ductile malleable cast iron according to different chemical composition, heat treatment process, performance and organization. More than 90% of the malleable cast iron produced in China is black heart malleable cast iron. The other three types of malleable cast iron have fewer applications. Black heart malleable cast iron is not strong, but has good plasticity and toughness. Malleable cast iron is mainly used in automobile rear axle axle housings, steering mechanisms, low-pressure valves, pipe joints and other parts subject to shock and vibration.

Malleable cast iron properties

The processing performance of white iron is extremely poor, but after high temperature tempering, it has higher strength and plasticity and can be cut.

Because the graphite in the malleable cast iron is flocculent and has a small effect on the matrix, its mechanical properties are higher than gray cast iron, and its plasticity and toughness are good, but the malleable cast iron cannot be forged. The matrix structure of malleable cast iron is different, and its performance is also different. Among them, black heart malleable cast iron has higher plasticity and toughness, while pearlite malleable cast iron has higher strength, hardness and wear resistance.

Malleable cast iron grades and uses

Malleable cast iron grades

The grades of malleable cast iron consist of "KTH" (" " three-letter Chinese pinyin prefix) or "KTZ" (" " three-character Chinese pinyin prefix) followed by the lowest tensile strength value (MPa) and the lowest Elongation after breaking is expressed as a percentage. For example, the grade KTH 350-10 indicates black core malleable cast iron with a minimum tensile strength of 350 MPa and a minimum elongation at break of 10%, that is, ferritic malleable cast iron; KTZ 650-02 indicates a minimum tensile strength of 650 MPa and minimum elongation after breaking Pearlite malleable cast iron with a rate of 2%.

Common types of malleable cast iron

Grade, performance and use (GB 9440-1988)

Cast iron grades KTH30006, KTH33008, KTH35010, and KTH37012: used to manufacture pipe fittings, low-pressure valves, rear axle housings for automotive tractors, steering mechanisms, machine tool parts, and so on.

Cast iron grades KTZ450-06, KTZ550-04, KTZ650-02, KTZ700-02: Castings that require higher strength and better wear resistance, such as gear boxes, camshafts, crankshafts, connecting rods, piston rings, etc.

Cast iron grades KTB380-04, KTB380-12, KTB400-05, KTB450-07: This is a white-heart malleable cast iron, which is limited to the manufacture of thin-walled castings and castings that do not require heat treatment after welding. Due to the more complicated process, it is used in machinery manufacturing Less applied.

Designation method of malleable cast iron (2 photos)

Application range of malleable cast iron

Black heart malleable cast iron has low strength, hardness, good plasticity and toughness, and is used for parts with low load, high impact and vibration.

The pearlite matrix malleable cast iron has high strength and hardness, and is used for important parts with high load, wear resistance and certain toughness requirements.

Application of malleable cast iron Such as oil pipelines, refinery pipelines and pipe fittings for gas and water supply systems of commercial and civil buildings.

Development history of malleable cast iron

Tube History China is one of the countries with the longest history of producing malleable cast iron. As early as the early Warring States Period, a process of obtaining ductile cast iron by using heat treatment to precipitate carbon in the white cast iron with graphite was precipitated. The white iron rod with steel surface and annealed surface decarburized at the beginning of the Warring States Period unearthed in Luoyang, Henan was an example of an annealing operation at that time. Prolonging the annealing time on this basis can produce ductile (forgeable) cast iron. This-invention allowed cast iron to be used in large quantities and widely in military and agricultural production at that time. The "Mencius" records the words of Meng Xun (circa 390-305 BC), "Xu Zi cultivated with iron?" It reflects the popularization of cast iron farm tools in the 4th century BC. From 1720 to 1722, the Frenchman, Reaumur, invented the production method of white-heart malleable cast iron, which was later commonly referred to as "Procede europeen". In 1982, SethBoyden, an American, decomposed Fe3C in white cast iron by accidental heat treatment to precipitate a flocculent graphite + metal matrix (ferrite or pearlite). The malleable cast iron he obtained at the time was ferritic. This method is commonly referred to as "American law" (black heart malleable cast iron).

Malleable cast iron

White heart malleable cast iron, developed by the Frenchman RAF de Luo Mao in 1722. White cast iron with a low carbon and silicon content is sealed in an oxidizing medium, and it is maintained at a temperature of 950 to 1050 ° C for several tens of hours for decarburization annealing treatment to obtain a ferrite with an outer layer and a small amount of pearlite remaining in the center. And the microstructure of flocculent graphite. The heart fracture is white, so it is called white heart malleable cast iron.

Malleable iron

Black heart malleable cast iron was developed by American S. Boyden in 1826 based on the introduction of white heart malleable cast iron. After the white cast iron with low carbon and silicon content is placed in a neutral medium for graphitization treatment, it is kept at a temperature of 850 to 950 ° C for several tens of hours, and the furnace is cooled to 720 to 740 ° C for another ten hours, and finally A ferritic black core malleable cast iron having a ferrite matrix and flocculent graphite is obtained; or after being held at 850 to 950 ° C for more than ten hours, the furnace is released and cooled in the air to obtain a pearlite black core malleable cast iron having a pearlite matrix and flocculent graphite. Properties and uses The graphite in the malleable cast iron is flocculent, with less content, and the phenomenon of stress concentration is not significant. The effective load area of the cast iron is not reduced, the tensile strength can reach 300 700 MPa, and the elongation can reach 2 12%, good cutting performance, oxidation resistance and corrosion resistance. The as-cast structure of malleable cast iron is white, the molten iron has poor fluidity, is prone to shrinkage, and has a large tendency to hot crack, so it is generally only suitable for castings with less complicated shapes. In addition, since the annealing time is prolonged with the increase of the wall thickness, and the central part of the excessively thick casting is difficult to achieve complete annealing, the wall thickness of the white-heart malleable cast iron generally does not exceed 12 mm, and the wall thickness of the black-heart malleable cast iron does not exceed 25 mm. Ferritic malleable cast iron is widely used in automobile, tractor wheels, differential housings and chassis parts, wrenches in machine tool accessories, porcelain bottle iron caps in power transmission lines, clamps, bowl head plates, and woolen machines in textile machinery And elbow, tee, joint, medium pressure valve in water and oil pipeline of printing machine. Pearlite malleable cast iron is used in valve rockers, coal feeder parts, high-pressure joint valve bodies, shift forks in the automotive industry, and differential gear boxes. White heart malleable cast iron is used in automobile parts hangers, steering column fork shoulders, textile machine parts, etc. Research on the development of malleable cast iron mainly focuses on as-cast stable carbides, free graphite flakes do not appear in castings, shortening the annealing time to improve mechanical properties and serviceability. In addition, the scope of casting thickness and weight restrictions has been expanded. Hyundai has produced malleable cast iron with a wall thickness of 2 to 80 mm or a weight of 150 kg.

Features of malleable cast iron

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

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

(1) Mechanism of solid graphitization. The cementite in the white mouth green body is an unstable phase and can be decomposed into stable phases as long as conditions are met-ferrite and graphite. This 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 the perspective of thermodynamics, cementite is lower than the iron-carbon phase diagram A. If it is kept under many temperature conditions, solid state graphitization can also occur. However, whether the decomposition of cementite can continue and whether the graphitization process can be finally completed depends to a large extent on the ability and possibility of the diffusion of carbon atoms after the decomposition of cementite, so that the old phase disappears and the new phase is formed. Kinetic conditions such as resistance factors. In the presence of multiple phases of cementite and matrix, graphite crystal nuclei are most likely to be generated at the interface between cementite and surrounding solid solution; if there are various particles of inclusions such as sulfides and oxides in the cast iron, graphite crystal nuclei Formation is relatively easy. In order for the graphite nuclei existing in white cast iron to continue to grow, it must have the conditions for carbon atoms to diffuse strongly. Pure iron-carbon alloys are more difficult to graphitize. When there are elements that promote graphitization, they can accelerate the graphitization process. Many opinions on the solid-state graphitization mechanism of cast iron are mostly based on the traditional two-stage annealing process. At the high temperature stage, when heated to the austenite temperature region, it passes through four links: nucleation at the austenite-cementite interface; cementite is dissolved in the surrounding austenite; carbon atoms are in the austenite Diffusion from the austenite cementite interface to the austenite-graphite interface; precipitation of carbon atoms on the graphite core leads to graphite growth. During this stage of annealing,. The cementite continues to dissolve, and the graphite continues to grow until the cementite is completely dissolved. At this time, the equilibrium structure of cast iron is austenite plus graphite. At the low temperature stage, a eutectoid transformation into ferrite occurs, and finally a balanced structure of ferrite and graphite is formed. Due to the advent of low-temperature graphitization annealing, solid-state graphitization mechanisms have evolved. The heating temperature is not higher than A, the temperature is only 720 750 , the cast iron structure is directly transformed from the original pearlite and laisite to ferrite and graphite. The key is to improve the dynamic conditions of graphitization at lower temperatures and to strengthen the intrinsic graphitization factors of cast iron. Such as refining cementite, refining the grains to increase the interface and increase the dislocation density, thereby increasing the initial number of graphite cores to reduce the diffusion distance.

(2) Influence of graphitization annealing process. In the first stage, the temperature is usually 920-980 ° C, and the eutectic cementite in the perlite is gradually dissolved into the austenite and gradually disappears, and the flocculent stone is gradually formed. The commonly used temperature in the second stage is 710 to 730 ° C for heat preservation, or slowly (3 to 5 ° C / h) from 750 ° C to 700 ° C. The pre-treatment temperature is usually divided into high-temperature pre-treatment, that is, holding at about 750 ° C for 1 to 2 hours, and low-temperature pre-treatment, that is, holding at 350-450 ° C for 3 to 5 hours. Its role is to increase the number of granite particles, reduce the distance of carbon atom diffusion, shorten the annealing cycle, and improve the shape of graphite.

(3) The effects of various elements on solid graphitization. Carbon can promote stone greening, increase the number of graphite cores annealed, and shorten the stone optimization time, especially the graphitization time in the second stage. 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 compound inoculant in front of the furnace can cause large concentrations to fluctuate, which is conducive to low-temperature graphitization. Manganese can generate MnS with sulfur, so the graphitization time can be shortened within the appropriate content range. However, when the amount of free manganese (the amount of excess manganese other than MnS combined with sulfide) exceeds-a fixed value (> 0.15% to 0.25%) or is insufficient (negative value), it hinders graphitization, especially the second stage graphite Into. Sulfur strongly hinders graphitization. When the sulfur content is not very high (<0.25%), manganese can be used to neutralize its harmful effects. When the sulfur content is high, it becomes difficult to anneal the graphitization. Phosphorus weakly promotes graphitization during solidification, and has little effect on solid 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. all have a strong resistance to graphitization; aluminum, zirconium, and calcium have a strong role in promoting graphitization. ^[1]