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17-4 PH Stainless steel Investment Casting

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What is 17-4 PH stainless steel?

17-4PH stainless steel is an austenitic stainless steel containing more than 17% chromium, and its performance is equivalent to that of 18-8PH. This steel is used to make corrosion-resistant equipment, stainless steel for the food industry and general purposes, such as chemical processing, petroleum refining units, seawater treatment systems, and nuclear reactors. It has high strength and hardness at high temperature (tensile strength can reach 1000-1200 MPa), and it has high strength and elasticity at low temperature (elongation can reach up to 50%). Due to the high chromium content of this stainless steel, the mechanical properties of 17-4ph steel are very close to those of 18-8ph steel; at the same time, it also contains a certain amount of nickel, so it has better corrosion resistance than 18-8ph steel.

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About 17-4 PH casting grade

In the casting production process, due to the shrinkage of metal, thermal cracking and process hardening and other effects, so that the castings produce internal stresses. When the internal stress is greater than the yield limit of the metal at room temperature (that is, the material modulus of elasticity E less than or equal to 0.80MPa), it will produce compressive stress inside the casting; when the internal stress is greater than the yield limit of the metal at high temperature (that is, the material modulus of elasticity E>=1.20MPa), it will produce tensile strain. This change in volume due to temperature changes is called thermal and tissue stress. Thermal stresses and tissue strains are one of the main factors causing defects in steel, and are also one of the main factors affecting the performance of steel. Therefore, in order to ensure that the steel has sufficient strength and toughness and good weldability and cold-forming properties and other comprehensive mechanical properties, the casting must be annealed to eliminate or reduce these harmful internal stresses.

Heat Treatment of 17-4PH Stainless Steel Precision Castings

Heat treatment is an important process in the production of 17-4PH stainless steel precision castings, heat treatment is good or bad directly affect the performance and service life of the product. In general, 17-4PH stainless steel precision castings in the thermal process, due to the role of high temperature and phase change, resulting in certain organizational changes and changes in mechanical properties. Therefore, the 17-4PH stainless steel precision castings must be annealed, normalized and other heat treatment processes to eliminate the role of internal stress. The following austenitic-ferritic steel as an example of heat treatment process and operation points: 1, annealing The purpose of annealing is mainly to eliminate the effect of the mesh carburizing body in the eutectic steel, reduce the hardness of the eutectic steel, brittleness and improve the machinability and weldability. For the requirements of the parts (such as the support), can also be appropriately reduced by heating temperature or the use of slow cooling method so that parts to achieve the required hardness value; and for the higher requirements of the parts (such as crankshaft, connecting rod) is to use quenching + tempering with the method to meet the requirements of use. Annealing is generally carried out in a protective atmosphere (furnace temperature of 1040 ~ 1150 ° C or so). Commonly used insulation media are asbestos board, diatomaceous earth bricks, etc., according to the need for graphite powder is sometimes used as a thermal insulation layer.

The pros of stainless steel 17-4 PH

1, good corrosion resistance. 17-4PH stainless steel has very good corrosion resistance, it can not only resist corrosion in alkaline solutions and most acid solutions, but also in the reducing environment. For example, it can remain stable in concentrated hydrochloric acid at high temperatures and is resistant to the action of any kind of organic acid as well as non-oxidising salts. Due to this characteristic, 17-4PH stainless steel is often used as a substrate or lining material in chemical equipment. 2, high strength, good toughness, easy to weld. 17-4PH stainless steel strength is very high (yield strength up to 800Mpa), its toughness is also very good; In addition, 17-4PH ductility is also very large (elongation rate of 10%). Therefore, when hot working (above 17°C), its properties do not change, and when cold working it does not deform; in addition, it is also very weldable.

The cons of stainless steel 17-4 PH

1,Pitting resistance is poor, generally used for seawater corrosion resistance, good resistance to chloride ion permeability in the environment.

2, poor weldability. Due to the high carbon content in the steel, easy to form martensite organization and embrittlement (i.e., reduced hardness). Therefore, 17-4PH stainless steel on the weld heat affected zone toughness requirements are high, and not suitable for TIG welding.

3, cold working hardening serious.

4, the price is expensive.

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Application of stainless steel 17-4 PH in petrochemical industry

In the petroleum industry, PH stainless steel is mainly used to make various storage tanks, pipes and containers. As PH stainless steel has good corrosion resistance, strong mechanical properties and low price, so is widely used in the oil industry. The world’s largest oil storage tank is the Russian “Kuleba” storage tank (capacity of 10 million barrels), the material is 17-4PH steel. In addition, the “Aframax” refinery also uses a large amount of PH stainless steel to manufacture equipment and pipeline linings and other components.

Application of stainless steel 17-4 PH in nuclear power

Nuclear power is a clean, efficient, safe and reliable energy source, but nuclear power plants operate in a very demanding environment. To ensure the safe and reliable operation of equipment, stainless steel is used as a material for key components to achieve efficient cooling and sealing of the equipment. Stainless steel 17-4PH has excellent corrosion resistance, high temperature resistance and good processability.

Application of stainless steel 17-4 PH in Shipbuilding industry

In shipbuilding, PH stainless steel is mainly used in marine boilers, heat exchangers and condensers. This equipment often requires materials with good corrosion resistance and PH stainless steel is therefore the ideal material choice. The tensile strength of 17-4 PH steel reaches around 1000 MPa, while the tensile strength of ordinary austenitic stainless steel is only around 400 MPA.

17-4PH Stainless Steel  

Application of stainless steel 17-4 PH in medical industry

In hospitals, stainless steel 17-4PH is used for surface decoration and cleaning of various medical devices. Such as scalpels, scissors, needles, catheters, drainage bags, etc.; can also be used for medical equipment sterilization cabinet door plate, the shell of medical equipment, etc.

17-4 PH Will there be rust on investment castings?

In the casting process, the metal liquid in the pouring system of cooling solidification, shrinkage crystallization and surface hardening and other effects of the metal to produce a variety of defects. The presence of these defects will affect the quality of the casting. Therefore, certain process measures must be taken to prevent or reduce the production of these defects, to improve the quality of the casting. For example: to control the temperature of the steel; control the content of impurities in the molten iron; the use of appropriate pouring system and riser position and other measures to prevent shrinkage (pinhole) and loose (porous) generation and eliminate shrinkage; the use of reasonable alloying elements and control the pouring temperature to improve the grain size and avoid over-burning; through the selection of suitable breeding process to improve the denseness, strength and wear resistance of cast iron parts.

In addition, attention should be paid to prevent the production of oxidation skin, slag, sticky sand and white mouth organization and other phenomena. Due to the above reasons caused by the surface of the casting rust stains called corrosion or electrochemical corrosion phenomenon. This electrochemical corrosion is mainly caused by the oxygen in the air and the chemical reaction caused by the contact with the metal. (1) air is one of the main corrosion factors, it includes atmospheric pollution gas (oxygen), soot particles and humid air environment; (2) water contains a large number of oxygen ions; (3) some materials themselves have hygroscopicity; (4) some materials at high temperatures prone to decomposition reaction; (5) some materials are easy to adsorb moisture or absorb moisture and reduce its corrosion resistance; (6) some equipment in the use of process will produce acidic gas, etc.

What problems can arise during the process of casting 17-4PH stainless steel?

1. As the casting is cooled at high temperatures, thermal stresses are generated. When the temperature is higher than 800°C, the thermal stress will increase sharply; while when the temperature is lower than 400°C, the thermal stress will drop rapidly. So for the large size of the casting, 17-4PH stainless steel solidification speed to slow some, so that the internal heat generated enough for tissue transformation (i.e. austenitization).

2. Due to the role of alloying elements and the influence of gas and inclusions in the steel and the hindering effect in the pouring system can lead to different shrinkage rates of castings. In general, large or heavier 17-4PH stainless steel solidification time is longer (about 20 ~ 30 minutes), and the shrinkage rate is also larger; and small or lighter alloy is relatively short (about 10 ~ 15 seconds) and shrinkage rate is smaller (17-4PH stainless steel has a large coefficient of linear expansion)

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