Optimization of heat treatment process for Cr8-type edge tool steel
In the 1980s, domestic and foreign have developed Cr content of about 8% of the Cr8 type cold work die steel (referred to as Cr8 steel), such as BOHLERK340, K360, SDC99, etc.. This type of steel contains about 8% Cr, adding Mo, V, Si and other alloying elements, its carbide is small and uniformly distributed, widely used in impact resistant molds, finishing molds and sharpening tool materials.
The test material is Cr8 steel, which is used as cutting tool for steel plate in a company. In the cutting process, the cutting tool is subjected to strong friction of the workpiece, which requires high hardness and wear resistance; at the same time, the cutting edge is subjected to different loads such as bending, twisting, shearing and impact, and the material needs sufficient toughness. The heat treatment process is the key to obtain excellent performance. In this work, the effects of quenching temperature, tempering temperature, tempering time, tempering frequency and tempering cooling method on the organization and performance are explored to improve the toughness of this material under the condition of ensuring high hardness, thus extending its service life as an edge tool.
The chemical composition (mass fraction, %) of the material was measured using a mobile spectrometer as follows: C 0.508, Cr 7.51, Mo 2.42, Si 0.732, V 1.98, Mn 0.290, and Fe residual. The metallurgical process of the test material is: ex-works condition - homogenized annealing - spheroidal annealing - heat treatment (quenching + tempering). Heat treatment process in the box resistance furnace, quenching specimen size of 10mm × 10mm × 15mm, the quenching temperature of the choice of 950 ~ 1200 ℃, in the quenching temperature holding 15min oil quenching, quenching organization with 2% hydrochloric acid alcohol solution corrosion. Tempering test for quenched steel 200 ~ 600 ℃, tempering 2 h. Select the quenching and tempering temperature with the highest hardness after heat treatment, and discuss the effect of tempering time, tempering number and tempering cooling method on hardness and impact properties under the premise of ensuring high hardness. The impact specimens were made of unnotched specimens with dimensions of 7mm×10mm×55mm. microstructure was observed by scanning electron microscopy, hardness and impact absorption energy were tested by Rockwell hardness tester and impact tester, physical phase analysis was performed by X-ray meter, and residual austenite content in the specimens was quantified by X-ray stress tester. The results showed that.
(1) Cr8 steel quenching organization is martensite, residual austenite and carbide. Low-temperature quenching gives slate-martensite, and high-temperature quenching gives pinning martensite.
(2) with the increase in tempering temperature, low-temperature quenching specimens without secondary hardening peaks; and high-temperature quenching specimens tempered at 520 ℃ or so there are obvious secondary hardening peaks, so choose 1100 ℃ quenching + 520 ℃ tempering.
High-temperature quenching and high-temperature tempering specimens with the number of tempering to increase the impact absorption energy first decreased and then increased, and tempered oil cooling than air-cooled hardness of 1 to 2 HRC. the final heat treatment process for 1100 ℃ oil quenching + 520 ℃ × 2h tempering 2 times, oil cooling.