October 25, 2020

Casting materials and their melting

What China has been using for many years is based on the carbon content of steel as a grading standard. Table 1 lists the national standards for cast carbon steel. Regarding the grade, chemical composition and mechanical properties of the steel, the “ZG” in the grade indicates the cast steel, and the subsequent figures indicate the nominal value of the weight fraction of carbon in the steel. Expressed in parts per million. Cast carbon steel is classified into three grades according to the phosphorus and sulfur content of its impurity elements, and the grades of grades of phosphorus and sulfur are less than 0.04% (I grade) steel; and the quality of grade II grade steel is less than 0.05%. Less than 0.06% is ordinary (III grade) steel.

The general engineering cast carbon steel standard (GB5676-85) divides the cast carbon steel into five grades according to the mechanical properties at room temperature, namely ZG200-400, ZG230-450, ZG270-500, ZG310-570 and ZG340-640. The basic chemical composition of steel is only specified as the upper limit of its mass fraction, and the restrictions on residual alloying elements in steel are relatively wide.

2, casting low alloy steel

2.1 General casting low alloy steel series steel

In mechanical manufacturing, general-purpose cast low-alloy steels mainly include three series of manganese, chromium and nickel. These series of steels are alloyed on the basis of the composition of the cast carbon steel and are subjected to phase heat treatment to obtain higher normal temperature mechanical properties than cast steel.

1) Manganese low alloy steel: manganese is used as the main alloying element, and silicon, molybdenum and the like are used as auxiliary strengthening elements to constitute manganese steel, manganese silicon steel, manganese silicon chromium steel and manganese molybdenum steel.

2) Chromium-based low-alloy steel: chromium is used as the main alloying element, and molybdenum, nickel, etc. are used as auxiliary strengthening elements to form chrome steel and chrome-nickel steel.

3) Nickel-based low-alloy steel: nickel is used as the main alloying element, and chromium steel or auxiliary element is used to form nickel steel, nickel-chromium steel, nickel-chromium-molybdenum steel.

2.2 Low alloy steel grades with special properties and uses

According to the special performance requirements for castings, the alloy design of steel is a cast low-alloy steel with special purpose, including precipitation-strengthened low-alloy steel for thick section and no quenching treatment. Low-alloy steel for heat, low-alloy steel for low temperature, and low-alloy steel for anti-wear.

3. Casting high alloy steel

In casting high-alloy steel, the total amount of alloying elements added is 10% (mass fraction) or more, and the alloying elements may be one, two, or more. After the steel contains a large amount of alloying elements, the organization has undergone fundamental changes. The steel has special performance properties, such as austenitic high manganese steel with ωMn=13%, which has high impact wear resistance, and austenitic stainless steel such as ωcr=18%, ωNi=, which has good performance. Corrosion resistance, etc. Therefore, high alloy cast steel is actually a special cast steel.

Compared with special cast iron, high alloy cast steel has higher performance, especially mechanical properties, such as high chromium anti-wear white cast iron, although it has high wear resistance, but its toughness is poor, not suitable for high Working under the influence of impact force, high manganese steel has both high wear resistance and high impact toughness, and can withstand high impact wear. Another example is that high-silicon cast iron is strong and corrosive in acid-based aggregates, but its strength is very low and it is extremely brittle. Austenitic stainless steel is both corrosive, high strength and high impact toughness. It is suitable for corrosion resistant castings subjected to shock or vibration conditions, such as the acid pump's wheel. For example, high-chromium cast iron has high heat resistance, but it is also a low-strength, high-brittle material, while high chromium nickel steel and chromium manganese nitrogen steel have high strength and toughness. Therefore, high alloy cast steel bit cast iron is more suitable for machine parts that work under heavy load, shock and vibration conditions. Bit cast iron has greater reliability and safety.

Because high alloy steel contains a large amount of alloying elements, it is inferior to carbon steel and low alloy steel in terms of casting properties, weldability and cutting performance. In terms of casting properties, each high-alloy steel has its own characteristics, and it is necessary to formulate a corresponding casting process according to its casting performance characteristics. In terms of welding, it is generally necessary to use a specific alloy electrode. Some steels need to be protected by inert gas during welding. It is also necessary to carry out preheating of the castings before welding and heat treatment to improve the welding site and the welding stress after welding. In terms of cutting, due to the high hardness of high-alloy steels, some steels have high toughness, so that the tools and cutting tools used to process general carbon steel and low alloys cannot be processed, but must be cut with specific tools. Process.

3.1 Cast anti-wear steel (high manganese steel)

Among cast anti-wear steels, high manganese steel is the most versatile one. The nominal content of manganese in high manganese steel is 13% (mass fraction), and the grade is ZGMn13. The steel has a single austenitic structure after heat treatment, and the toughness is very good, but the hardness is not high, but the austenite has processing hardness. When the casting is subjected to strong impact or extrusion during work, the surface layer structure is work hardened, the hardness is greatly improved, and thus the wear resistance is high.

3.2 Cast corrosion resistant steel (stainless steel)

There are many steels for casting corrosion-resistant steel, but basically all use chromium as the main alloying element for corrosion resistance. According to chemical composition and organization, it can be divided into two types: chromium stainless steel and chromium nickel stainless steel.

1) Chrome stainless steel

The nominal content of chrome stainless steel is 13%, which is the lowest chromium content among stainless steel grades. Cr13 steel is a series, divided into five steel grades according to different steel contents, namely 0Cr13, 1Cr13, 2Cr13, 3Cr13 and 4Cr13. As the cast stainless steel, two kinds of ZG0Cr13 and ZG1Cr13 with good corrosion resistance are used.

2) Chrome nickel stainless steel

The nominal content of chromium in chrome-nickel stainless steel is ωcr=18%, and its corrosion resistance is better than that of Cr13 steel.

3.3 anti-wear and corrosion resistant alloy steel (stainless steel)

Due to the needs of hydropower and other industries, anti-wear and corrosion-resistant alloy steels have been developed at home and abroad in recent years, typically chromium-nickel-molybdenum martensitic stainless steel and precipitation-hardened cast stainless steel for the manufacture of turbine rotors and single-blade blades. .

Anti-wear and corrosion-resistant stainless steel also uses chromium as the main corrosion-resistant alloy element. In order to make the steel have high hardness, the steel should have martensite structure. To this end, nickel and molybdenum are added to the steel so that under the combined action of chromium and nickel, the steel is still highly hardenable, so that large thick-walled castings can be obtained along the full section under oil quenching or open conditions. Thickness of martensite structure. In the precipitation hardening type stainless steel, since the dispersion hardened phase precipitates on the martensite matrix, the hardness and abrasion resistance of the steel are further improved. In order to ensure good corrosion resistance and weldability, the mass fraction of carbon in such steels is relatively low, generally below 0.1%.

3.4 cast heat resistant steel

The heat resistance is lower than that of the alloy steel at a temperature lower than 400 ° C and can maintain its strength, but at a higher temperature, heat resistance requires high alloy steel.

The high temperature properties of steel include oxidation resistance and heat strength. The oxidation resistance is the stability of steel to oxidative corrosion of gaseous medium at high temperature. The thermal strength is that steel can maintain mechanical load for a long time at high temperature. Ability. The steel grades used at high temperatures are classified into two categories according to these two properties.

1) Heat-resistant stainless steel (anti-oxidation steel), which is a steel with good oxidation resistance at high temperatures, but has no strict requirements on the high-temperature strength of steel.

2) Heat-strength steel, which is a steel that has both good oxidation resistance at high temperatures and high strength for a long period of time.

There are four types of steels for heat resistance temperature above 800 °C used in production, such as chrome heat resistant steel, chrome-nickel heat-resistant steel, chrome-manganese nitrous heat-resistant steel and aluminum-manganese heat-resistant steel.

3.5 casting heat-strength steel

1) Strength and thermal strength of steel at high temperatures

When steel is stressed at high temperatures, two phenomena occur, namely softening and creep. Softening is manifested by a decrease in strength and an increase in plasticity. The performance of creep is that when the steel is stressed at high temperature, the deformation amount increases under the condition of constant stress until the final fracture.

2) Low body heat steel

Heat-strength steels are usually classified into a pearlite type, a martensite type, and an austenite type according to their metallographic structure. The first two contain less strengthening elements and have lower heat strength, and are generally used at temperatures below 600 °C. Austenite can solidify a large number of alloying elements, which is beneficial to improve the thermal strength of steel. So at 600. Heat-strength steels used at temperatures above C are basically austenitic

The heat-strength material used at higher temperatures (above 800 °C) is no longer a steel (iron-based alloy) but a nickel-based alloy or a cobalt-based alloy.

4. Melting of cast steel

4.1 Purpose and requirements of steelmaking

The purpose and requirements of steelmaking include the following four aspects:

1) Melt the charge into molten steel and increase its superheat temperature to ensure the need for pouring.

2) The content of silicon, manganese and carbon (including alloying elements in the case of alloy steel) in the molten steel is controlled within the regular range.

3) Reduce the harmful elements of sulfur and phosphorus in the molten steel, so that the total content is reduced below the specified limit.

4) Remove non-metallic inclusions and gases from the molten steel to make the molten steel pure.

4.2 Steelmaking methods, characteristics and applications

1) Electric arc furnace steelmaking

The basic structure of the electric arc furnace is shown in Figure 1. The heat generated by the arc is used to melt the charge and increase the temperature of the molten steel. Since the electric arc furnace is heated without the method of fuel combustion, it is easy to control the properties of the furnace gas. It can be made oxidizing or reducing according to the requirements of smelting. Electric arc furnaces have become the most common steelmaking furnaces used in cast steel.

2) Induction furnace steelmaking

Steelmaking uses a coreless induction furnace, which works in the same way as an electric furnace that constructs cast iron. The structure and appearance of the furnace body are shown in Figure 2, which mainly includes two parts: the inductor and the cymbal. However, since steelmaking requires more heat, it is larger in terms of input power than furnaces of the same capacity. The induction furnace for steelmaking uses different frequencies depending on the furnace capacity (坩埚 diameter), the high frequency (10000 Hz or more) with a capacity of about 10 kg, and the intermediate frequency (1000 to 3000 Hz) with a capacity of 100 to 500 kg. The induction electric furnace of 500 kg or more uses an industrial electric frequency (50 Hz).

3) Open hearth steelmaking

The structure of the open hearth is shown in Figure 2. Using gas or heavy oil as fuel, mixing with preheating air supply, burning, the generated flame is directly sprayed on the charge, heating and melting. Because it is heated by flame, the furnace gas is oxidized, and the elements in the steel making process The burning loss is heavier than that of the electric furnace, and the capacity of the open hearth is large, generally ranging from several tens of tons to hundreds of tons, and is suitable for pouring type-oriented castings.

4) Ladle refining furnace

The furnace is melted in an electric arc furnace, and then the molten steel is poured into a ladle refining furnace (Fig. 3).

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