42CrMo4 QT-42CrMo4 çeliği için söndürme ve temperleme işlemi

The “QT” in 42CrMo4 QT stands for söndürme and tempering. 42CrMo4 steel is designated as 1.7225 in the UNS system and is a widely used high-strength alloy steel.

The equivalent grades of 42CrMo4 include:

1. USA (ASTM/AISI): AISI 4140, SAE 4140H, ASTM A193 B7.
2. Japonya (JIS): SCM440, SCM440H
3. Çin (GB): 42CrMo
4. İngiltere (BS): 708M40, EN19A
5. Fransa (AFNOR): 42CD4

42CrMo4 steel exhibits deep sertleştirilebilirlik, excellent comprehensive mechanical properties, and a low level of significant temper kırılganlık. It is extensively used in machinery components, fasteners, crankshafts, gears, and molds.

The standard heat treatment for 42CrMo4 often consists of quenching and tempering (Q&T), which involves heating the steel to form austenite, followed by rapid cooling and then reheating to a lower temperature.

42CrMo4 Steel Quenching Process

Quenching is the process of rapidly cooling a steel object from its austenitizing temperature to transform some or all of the austenite into martensite, which is a very hard and strong, but brittle, microstructure. At the same time, it can reduce residual stress and deformation of the material.

The austenitizing temperature of 42CrMo4 steel ranges from 800 to 880°C (1475 to 1650°F). Soaking time usually depends on the thickness of the material’s cross-section, with a minimum of 1 hour or 15 to 20 minutes per 25 mm (1 inch) of maximum section thickness.

1. Oil Quenching. 42CrMo4 steel is usually oil quenched. Oil quenchants can be classified as fast, intermediate, or slow, depending on their cooling rate and additives. Normal operating temperatures for oil quenchants range from 25 to 70 °C (80 to 160 °F). Hot oil can be used at temperatures as high as 175 °C (350 °F) to minimize distortion.
2. Water Quenching. While 42CrMo4 is typically oil-quenched, water quenching can be used for larger sections (e.g., round sections greater than 75 mm or 3 inches in diameter) to achieve through-hardening. Water quench baths are typically maintained at 20 to 40 °C (68 to 104 °F) and agitated to enhance cooling uniformity. However, water quenching significantly increases the possibility of cracking and distortion, especially at this carbon level.

42CrMo4 Steel Tavlama İşlemi

As-quenched martensite in steel is very hard but also very brittle and contains high residual stresses. The purpose of tempering is to reheat quenched steel to below its lower critical temperature (Ac1) in order to increase its toughness and ductility, reduce hardness, and relieve internal stress.

The tempering temperature for 42CrMo4 steel is 175 to 705°C (350 to 1300°F), with a holding time of at least 1.5 to 2 hours. Different tempering temperatures determine the final hardness and strength of the material, so we can decide on the tempering temperature based on the hardness and strength required for the material. For “ultra-high” strength, tempering between 175 °C and 230°C (350°F and 450°F) is used, while for lower yield strengths (below 1380 MPa or 200 ksi), temperatures between 385 °C and 705°C (725°F and 1300°F) are applied.¹ Cooling after tempering can be by air or water quenching.

Avoid tempering 42CrMo4 material at temperatures between 250 and 300°C (480 and 570°F). Tempering at these temperatures causes a decrease in impact toughness in 42CrMo4, a phenomenon known as Tempered Martensite Embrittlement (TME).

Özet

The mechanical properties of 42CrMo4 steel are highly dependent on the oil quenching and tempering at various temperatures. Generally, as the tempering temperature increases, hardness, tensile strength, and yield strength decrease, while ductility, toughness, and impact energy increase.

1. ASM Uluslararası. (1991). ASM El Kitabı, Cilt 4: Isıl İşlem (pp. 500-501). ASM International. ↩︎