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Eternal
Mold factories that make molds for PVC pipe fittings, flame-retardant electrical appliance housings, and transparent cosmetic packaging almost all face the same nightmare: after fewer than 20,000 shots in production, the cavity surface starts to show pitting and rust spots. The mirror finish slowly turns cloudy. Scratches and flash on the product surface come one after another. Next come line shutdown, mold repair, polishing, and putting the mold back into production—every cycle eats away profit.
The root cause is not the workers. Common mold steels (such as P20 and 718) have almost no defense against hydrogen chloride gas from PVC breakdown, hydrogen bromide released by flame retardants, and acidic corrosive substances from fluorine-containing plastics. Every time the cavity surface touches molten plastic, it is chemically attacked once. The corrosion pits then become “hiding places” for plastic residue, speeding up the next round of corrosion.
To solve this problem, we need a change at the material level. 3Cr17Mo is our answer—a martensitic stainless steel mold steel designed for corrosive plastic molding conditions. It is supplied as forgings and meets three needs at the same time: corrosion resistance, wear resistance, and mirror polishing.
3Cr17Mo is a corrosion-resistant plastic mold steel specified in China's national standard GB/T 1299. Its microstructure is martensitic stainless steel. Because its composition is very close to the German DIN grade 1.2316 (X36CrMo17), it is also often called "domestic 2316" in the Chinese market. The main international equivalents are as follows:
Standard | Grade | Note |
GB/T 1299 | 3Cr17Mo | Former brand name SM3Cr17Mo |
DIN/EN | 1.2316（X38CrMo16） | The international grade that is most similar in composition |
AISI/ASTM | 420F modified form | Equivalent substitution |
JIS | SUS420J2 | Approximate correspondence |
ISO | X38CrMo16 | corresponding ISO 4957 |
For users who previously used imported 1.2316, 3Cr17Mo forgings can directly replace it in composition and performance, while offering shorter lead times and lower cost.
Root cause analysis: PVC decomposes hydrogen chloride (HCl) in the molten state, and flame-retardant ABS releases hydrogen bromide (HBr). These acid gases form an electrolyte environment on the cavity surface. Ordinary mold steel does not have passivation ability, so corrosion continues at the micro-electrochemical level.
3Cr17Mo’s solution: The steel’s chromium content of 15.50%–17.50% forms a dense Cr₂O₃ passive film on the cavity surface. Molybdenum (0.80%–1.30%) further stabilizes the passive film and inhibits the start of pitting. In a neutral salt spray test, 3Cr17Mo can achieve 96 hours without red rust, and its corrosion rate in a 10% hydrochloric acid solution is below 0.05 mm/year. Academic research from the Central Iron and Steel Research Institute further confirms that the addition of molybdenum reduced the corrosion rate of similar martensitic stainless steel by about 55%, and an obvious passivation region appears on the polarization curve.
Actual results: A customer making medical injection molded products reported that previously, when using several corrosion-resistant mold steels to injection mold rigid PVC products, the molds still kept rusting, and electroplating treatment had little effect. After switching to a material with qualified corrosion resistance, the mold rusting problem was fundamentally solved.
Root cause analysis: When mirror polishing, foggy patches, local gloss differences, or hidden grinding marks appear. The root cause is often not the polishing process, but inclusions and hardness segregation inside the steel.
3Cr17Mo’s solution: Forgings from Jiangyin Eternal Heavy Industry Co.,Ltd use the electroslag remelting (ESR) process. This effectively reduces non-metallic inclusion content and gives a uniform, dense structure. After fine polishing, it can reach SPI A1 surface, with roughness Ra ≤ 0.05μm, meeting the optical-grade requirements for high-end applications such as optical lenses and transparent packaging. Forging further eliminates directional segregation that may come from rolling, ensuring consistent performance of large cross-section blocks.
Root cause analysis: After mold surface texturing (etching), mottled patterns with different depths appear. This is usually related to local segregation in the material and residual machining stress. Stress concentration areas react at different rates with the etching solution, causing uneven patterns that cannot be repaired by later processing.
3Cr17Mo’s solution: Forgings from Jiangyin Eternal Heavy Industry Co.,Ltd use forging ratio control and homogenizing annealing to refine grains and eliminate banded structure segregation. For large blocks over 200 mm thick, we recommend stress relief annealing after rough machining to release cutting stress and provide a uniform base structure for later texturing and mirror polishing.
Element | C | Si | Mn | Cr | Mo | Ni | P/S |
Range | 0.33~0.45 | ≤1.00 | ≤1.50 | 15.50~17.50 | 0.80~1.30 | ≤1.00 | ≤0.030 |
Item | Annealed | Pre-hardening | Q+T |
Hardness | ≤285HB | 30~38HRC | 46~48HRC |
Tensile strength | — | — | ≥735MPa；UP to 1400MPa |
Yield strength | — | — | ≥540MPa；UP to1200MPa |
Impact toughness | — | — | ≥20J/cm² |
Annealing: Heat to 800–900°C, then slow cool. Hardness drops to ≤285 HB, making machining easier.
Quenching: Oil cool from 1000–1040°C to obtain a structure of lath martensite + a small amount of ferrite + fine carbides.
Tempering: Low-temperature tempering at 260–300°C gives a high-hardness, wear-resistant state of about 48 HRC; high-temperature tempering at 550–600°C gives a pre-hardened state of 33–38 HRC. In production practice, double tempering is recommended to ensure structural stability.
The above quenching and tempering process parameters and microstructure characteristics have been verified by Jiangyin Eternal Heavy Industry Co.,Ltd in forging practice: after quenching at 1000–1060°C and tempering at 260–300°C or 550–600°C, 3Cr17Mo steel can achieve good overall mechanical properties. The quenched structure is lath martensite + ferrite + (Fe,Cr)₂₃C₆ carbides.
PVC pipe fittings, valves, and profile extrusion molds: resist HCl gas corrosion from PVC breakdown, with mold life 3–5 times longer than ordinary steel grades
Fluorine-containing plastic (PTFE) molding molds: withstand hydrogen fluoride corrosion
Flame-retardant plastic (flame-retardant ABS, flame-retardant PC) molds: resist corrosive gases released by brominated flame retardants
POM precision gears and structural parts molds: meet both corrosion resistance and wear resistance requirements
Cosmetic packaging molds (caps, housings, powder compact cases): SPI A1 mirror finish, ensuring flawless product appearance
Transparent cups, optical lenses, and LED lamp cover molds: high purity ensures optical-grade surfaces
Home appliance panels and automotive interior part molds: consistency and durability of high-gloss surfaces
Medical device plastic part molds: resistance to alcohol disinfection and cleaner corrosion
Hot oil pumps, valve plates, and valve bearings: use the high hardness and corrosion resistance after quenching and tempering
Food machinery blades and chemical pump/valve seals: resist corrosion from food acids and chemical media
Springs and precision shaft parts: maintain corrosion resistance under high-strength conditions
Large thick-wall blocks: forging eliminates rolling directionality, ensures uniform properties across thick sections, and is suitable for molds for large home appliance panels, automotive bumpers, etc.
Custom-shaped parts: can be forged into near-net shapes such as flanges, shafts, and rings, reducing machining allowance.
Heavy-duty mold inserts: such as sliders and ejector pins. The forged structure is dense and can withstand high clamping force and frequent mold opening and closing impact.
ESR + forging dual process guarantee: Steel ingots are refined by electroslag remelting (ESR) and then formed by forging. This gives low non-metallic inclusion content and a dense, uniform structure, providing a clean base for mirror polishing.
Full-process heat treatment control: From homogenizing annealing after forging to final delivery heat treatment, every step can be provided with process records and inspection reports.
Multiple delivery states: We can supply annealed (≤285 HB), pre-hardened (30–38 HRC), or quenched and tempered (46–48 HRC) material according to customer needs.
Flexible specifications: Forgings can be supplied as round bars, square blocks, mold blocks, rings, flanges, and other forms. Custom forging is supported.
Technical support: We provide full-process technical consulting, from material selection suggestions and heat treatment process parameters to polishing/texturing processes.
