718 mold steel is pre-hardened steel, pre-hardened state surface hardness of 290 ~ 330HBS. 718 mold steel is very high purity, not only has excellent balance and excellent polishability and photo-etching etching performance, but also has a high hardenability and good polishing performance, electrical processing performance and skin processing performance is also a similar material is more excellent. Imported 718 mold steel belongs to the pre-hardened mirror surface acid-proof plastic mold steel, without quenching can be used for mold cavities, flange can be flame hardened, can increase the hardness to 52HRC.

718 mold steel is suitable for:

①Large mold type plate, high surface requirements for household appliances.

②Applicable to large mirror surface plastic molds, such as automobiles, home appliances, audio and video products, etc.

③Can be used for plastic molds with mirror polish degree requirements, suitable for injection molds and blow molds of PA, POM, PS, PE, PP, ABS plastics.

④High polish degree and high requirement mold cavity.

Physical Properties
Expansion coefficient	20-100	20-200	20-300	20-400	20-500
10-6m/(m×K)	11.8	12.5	13.1	13.5	14.8
Heat transfer coefficient	20	200	300	/	/
W/m×K	31.2	34.3	34.1	/	/

Heat treatment:

Quenching temperature 840 ~ 870 degrees, tempering temperature 200 ~ 500 degrees, tempered hardness 50 ~ 55 HRC.

①Annealing: Under protected condition, heated to 700°C, evenly heated and cooled to 600°C in the furnace at 15°C/h, then air cooled.

②Stress relief: After rough machining, heat to 550°C, hold for 2 hours, cool slowly to 500°C, then air cool.

③Quenching and tempering:Before quenching, it must be fully annealed to eliminate machining stresses. Preheat at 500 ~ 600 ° C for 2 hours, 850 ° C quenching temperature for 30 minutes; to be heated and held in a protective atmosphere, to avoid decarburization and oxidation. Then cool in oil, or isothermal at 300°C for 4 minutes and then air cool. Cooling to 50 ~ 70 ° C, should be tempered immediately.

④Flame quenching and tempering: can make the mold surface hardness, so that the wear resistance to improve. The method is to use oxygen 2 acetylene flame (wind coal) in the mold surface heating, and then cooling hardening, hardness up to 45 ~ 52 HRC.

Treatment: preheat 150~200 HRC to prevent cracking; then heat the mold surface continuously to 850~950°C (the surface is light red), then air-cooling; when cooling to 50~70°C, temper immediately at 180~200°C to prevent cracking when grinding.

• Electric discharge machining

The final stage of fine discharge machining should be carried out with low current and high frequency. After discharge, the surface of the mold should be ground to remove the “EDM white layer”, and the mold should be tempered again at 500°C to remove the stress.

• Welding process

Welding of the mold should be avoided as much as possible, because welding increases the risk of mold breakage. If the mold must be welded, the mold should be preheated before welding, the preheating temperature is 200°C – 250°C. **Note: For large welding, stress relief is required after welding.

• Grinding Process

Proper grinding technique can avoid grinding cracks and improve the life of the mold. Low temperature tempered molds are particularly sensitive to grinding; grinding wheels must be properly sharpened and, where possible, softer, less bonded, open grain grinding wheels must be selected. Limit the maximum circumferential speed and use plenty of coolant.

Polishing process

Proper polishing techniques prevent cracks from occurring and increase the life of the tool. Materials that have undergone low temperature tempering are highly susceptible to cracking during grinding, when grinding wheels must be dressed and softer and open grain wheels are suitable, while adequate lubricant and proper circumferential speed are important.

• Photo-etching patterns

718 mold steel is particularly suitable for photo-etching due to its low sulfur content, which ensures a uniform and precise pattern on the mold surface.

• Carburizing treatment

1. Carburizing temperature is 850~940°C, and the time is determined by the depth of the product required.

2. If the carburizing temperature exceeds 880°C and the time exceeds 2 hours, normalization must be performed to maintain the fine grain size.

3. After carburizing, the product is cooled to 830°C in the furnace, quenched in oil after uniform heat, and then tempered.

4. If only high hardness is required but not fine grain size, quenching can be done at carburizing temperature above 880°C.

• Nitriding treatment

Nitriding treatment can get a hard surface layer, at the same time, have good wear resistance and corrosion resistance, the hardness after nitriding treatment in ammonia can reach 650HV. 570°C under soft nitriding treatment, can get the surface hardness of 700HV, after 2 hours of treatment, the depth of hardened layer is 0.01mm.

• Hard chrome layer coverage

After the tools to be covered with a hard chrome layer, they must be tempered at 180°C for 4 hours to avoid hydrogen embrittlement.