Stainless Steel Grade AISI 440C | EN 1.4125 | DIN X105CrMo17

AISI 440C | EN 1.4125 | DIN X105CrMo17 is a high-carbon martensitic stainless steel which, due to its high carbon, chromium and molybdenum content, has high strength, good hardness, corrosion resistance, wear resistance, and dimensional stability at high and low temperatures.

The steel is capable of achieving high strength, hardness and wear resistance among all stainless alloys after heat treatment. Hardness, HB type = 578 - 600 in the correct hardened condition. It is especially suitable for applications such as ball bearings and valve parts.

It is magnetic in both the pre-hardened and post-hardened condition. It has moderate corrosion resistance compared to other stainless steels. It exhibits better corrosion resistance in the hardened and tempered form.

Stainless steel AISI 440C is a member of the 400 series of stainless steels. It has the highest carbon content among them. The distinctive properties of this type of steel are its hardness, mechanical strength and fatigue resistance. This combination of properties makes this alloy useful in the production of cutting tools. Due to its high carbon content, it does not have outstanding corrosion resistance, but it can still be used for applications that require moderate corrosion resistance, such as surgical instruments.

AISI 440C stainless steel typically contains 78-83.1% iron, 16-18% chromium, 1-1.2% carbon, 1% (max) silicon, 1% (max) manganese, 0.8% molybdenum, 0.04% phosphorus and 0.02% sulfur. It has a martensitic grain structure. It is mainly hardened by solid solution strengthening of chromium and dispersion strengthening of molybdenum carbides.

Properties of stainless steel AISI 440C | EN 1.4125

AISI 440C stainless steel is one of four types of 440 stainless steel, including AISI 440A, AISI 440B and AISI 440F. Care must be taken to avoid over-tempering, which can reduce its corrosion resistance.

AISI 440C has a corrosion resistance that is somewhat similar to AISI 410, but lower than AISI 431, also lower than most ferritic stainless steels of the 400 series and all austenitic stainless steels of the 300 series. It has optimum corrosion resistance in the tempered condition when quenched below 400 °C. Hardening from 1090 °C will provide the best carbide solution and therefore the best corrosion resistance, but at this temperature a minimum soaking time should be allowed, otherwise excessive grain growth may occur. Polishing will further improve the corrosion resistance. The steel is not recommended for use in the annealed condition. It is essential that oxygen can always circulate freely over all stainless steel surfaces to ensure that the chromium oxide film is always present to protect the steel. If this is not the case, rusting will occur, as is the case with other types of stainless steel.

Forging

Steel should be heated to 760 °C - 820 °C, then slowly and evenly heated to 1050 °C - 1150 °C, held until the temperature is uniform throughout the area and forged immediately. Do not overheat, as this may result in loss of strength and ductility. Do not forge below 900 °C. Finished forgings should be cooled slowly in a furnace, heated with dry lime or ash to room temperature and immediately annealed.

Heat treatment

AISI 440C stainless steel is martensitic, which means that it is heat treated. The general goal of heat treating 440C stainless steel is to limit the amount of austenite in the steel as much as possible and to promote the formation of martensite. Heat treatment of this grade of steel usually occurs as follows:

Annealing

Full annealing is carried out in the temperature range from 850 to 900 °C, after which the material is slowly cooled in a furnace to approximately 600 °C and then cooled in air to room temperature. In addition, it can be heated to 735 - 785 °C (subcritical annealing) and then slowly cooled in a furnace to room temperature.

Tempering

The steel is heated to a temperature of 1010 - 1065 °C and then quenched in warm oil or air. Thicker sections should be quenched in oil to ensure a faster and more uniform temperature drop, while thinner sections can be quenched in air.

Quenching

Now the steel is tempered in the temperature range of 150 - 370 °C, where several levels of hardness and mechanical properties can be obtained. Hardening in the range of 425 - 565 °C will lead to a decrease in corrosion resistance and impact resistance, and tempering in the range of 590 - 675 °C leads to an increase in impact resistance and a decrease in hardness.

The latest cryogenic hardening methods for AISI 440C stainless steel have shown promise due to the greater success in reducing the austenite content of the steel. Deep cryogenic treatment and shallow cryogenic treatment are two methods that are used before the hardening process begins.

Deep cryogenic treatment involves cooling the steel from room temperature to -196 °C over 3 hours, holding it at that temperature for 24 hours, and then returning it to room temperature.

Shallow cryogenic treatment involves holding the steel at -80 °C for 5 hours and then allowing it to return to room temperature. After cryogenic hardening, the steel can be hardened at 200°C. Both methods have shown an increase in hardness of up to 7% and up to 4%, respectively.

Processing

The machinability of this grade of stainless steel is directly related to its hardness, so the optimum machining parameters vary considerably. Once the hardness is known, the machining parameters can be estimated. The machining parameters will vary depending on the structure/hardness of the steel. Such data can be used as a guide when machining steel with carbide tools.

Physical properties

Mechanical properties

Application

The unique properties of AISI 440C stainless steel make it useful for many applications. Its excellent wear resistance makes it suitable for the production of valve components and ball bearings with the requirements of the industry standard Rockwell hardness of 58. Its hardness and strength also make it suitable for the production of cutting tools such as knives and blades. It is also used to make measuring tools such as measuring blocks. It is not recommended for use at high temperatures (usually above 400 °C) to avoid a decrease in corrosion resistance and impact resistance.