Cr8 Type Cutting Tool Steel Heat Treatment Process

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In the 1980s, domestic and foreign developed Cr8 cold-work die steels (Cr8 steels for short) with a Cr content of about 8%, such as BOHLERK340, K360, SDC99, etc. This type of steel contains about 8% chromium, and is added with alloying elements such as Mo, V, and Si. Its carbides are fine and uniformly distributed, and are widely used in impact-resistant molds, finishing molds and cutting tool materials.

The test material is Cr8 steel, which is used as a cutting tool for steel plate in a certain enterprise. In the cutting process, the cutting tool is subject to strong friction from the work-piece, and requires high hardness and wear resistance; at the same time, the cutting edge is subjected to different loads such as bending, torsion, shear, and impact, and the material needs sufficient toughness. The heat treatment process is the key to obtaining excellent performance. This work is to explore the effects of quenching temperature, tempering temperature, tempering time, tempering times and tempering cooling methods on the structure and performance, and improve the material under the condition of ensuring high hardness. The toughness, thereby extending its service life as a cutting tool.

The chemical composition (mass fraction, %) of the material measured by a mobile spectrometer is: C 0.508, Cr 7.51, Mo 2.42, Si 0.732, V 1.98, Mn 0.290, Fe balance. The metallurgical process of the test material is: delivery state-homogenization annealing-spheroidizing annealing-heat treatment (quenching + tempering). The heat treatment process is carried out in a box-type resistance furnace. The size of the quenched sample is 10mm×10mm×15mm. The quenching temperature is selected from 950 to 1200 ℃. The quenching temperature is kept for 15 minutes for oil quenching. The quenched structure is corroded by 2% hydrochloric acid alcohol solution. The quenched steel is subjected to a tempering test at 200-600°C for 2h. Select the quenching and tempering temperature with the highest hardness after heat treatment. Under the premise of ensuring high hardness, discuss the influence of tempering time, tempering times and tempering cooling methods on hardness and impact performance. The impact sample is a non-notched sample with a size of 7mm×10mm×55mm. Observe the microstructure with scanning electron microscope, use Rockwell hardness tester and impact tester to test the hardness and impact absorption energy, use X-ray instrument for phase analysis, and use X-ray stress tester to quantitatively determine the residual austenite content in the sample.

Results show:

(1) The quenched structure of Cr8 steel is martensite, retained austenite and carbide. Low-temperature quenching obtains lath martensite, and high-temperature quenching obtains needle-like martensite.

(2) As the tempering temperature increases, there is no secondary hardening peak for the low-temperature quenched sample; while the high-temperature quenched sample has an obvious secondary hardening peak when tempered at about 520℃, so choose 1100℃ quenching + 520℃ tempering .

The impact absorption energy of the high temperature quenched and high temperature tempered samples decreases with the increase of the number of tempering, and the hardness of oil cooling is 1～2HRC higher than that of air cooling after tempering. The final heat treatment process is 1100℃ oil quenching + 520℃×2h tempering twice, oil cooling.

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