Tham khảo tài liệu 'asm metals handbook - desk edition (asm_ 1998) ww part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | indicates that yield and tensile strengths are virtually identical from room temperature to 600 C 1100 F but that ductility is moderately lower in cast material. Hot hardness of cast H13 is higher than that of wrought H13 at temperatures above about 300 C about 600 F this hardness advantage increases with temperature and measures about eight points on the HRC scale at 650 C 1200 F . Because cast dies exhibit uniform properties in all directions no problem of directionality anisotropy exists. Dimensional control of castings is very consistent after an initial die is made and any necessary corrections are incorporated in the pattern. Reasonable finishing allowances are to mm to in. on the impression faces to mm to I I in. at the parting line of the mold and to mm I I to in. on the back and outside surfaces. The hot-work tool steels most commonly cast include H12 H13 H21 and H25. Specialty Tool Steels Through-Hardening Stainless Steels. Type 420 martensitic stainless steel and modifications of this alloy is commonly used for injection molds for all thermoplastic materials. It is particularly adaptable for molding vinyls or other corrosive plastics or when the atmospheric or storage conditions are unusually severe because it does not require chromium plating to resist these types of corrosive attack. Other stainless steels used for plastic molds include type 414 free-machining grade 420F and Elmax a high-hardness 58 to 60 HRC wear-resistant P M grade. Chemical compositions of these stainless steels are given in Table 8. Table 8 Chemical compositions of martensitic stainless steel plastic mold materials Alloy Composition wt C Si Mn Cr Ni Mo V S 414 . . . 420 mod . . . 420F mod . . . Elmax a . . a P M stainless steel produced by hot isostatic pressing of gas-atomized stainless steel powder Type 440C martensitic stainless steel both in wrought
