H13 Mold Steel Nitriding Treatment: Process Parameter Control & Failure Mode Analysis for Aluminum Casting Molds
Nitriding treatment improves surface hardness, anti‑sticking and wear‑resistance for H13 mold. Nitriding temperature, holding time, pre‑nitriding heat‑treatment status and surface preparation determine nitriding‑layer quality for LPDC, gravity and CPC counter‑pressure casting molds.
Nitriding layer structure composition: compound layer plus diffusion layer. Compound layer provides anti‑aluminum‑sticking performance; diffusion layer supports hardness gradient to avoid layer peeling under cyclic thermal‑mechanical load for EV structural‑part mold.
Target layer specification for aluminum casting mold: total nitriding depth 0.12‑0.20 mm; compound‑layer thickness controlled within 5‑12 μm. Over‑thick compound‑layer brings brittleness risk under thermal‑cycling for CPC counter‑pressure casting mold.
Pre‑nitriding requirement: mold substrate must complete quenching‑and‑tempering. Un‑tempered or decarburized substrate cannot obtain qualified nitriding layer; surface decarburization cannot be eliminated merely by subsequent nitriding for gravity casting mold.
Nitriding‑temperature window: 490‑530 ℃. Temperature below 480 ℃ results in too‑low nitriding rate; temperature above 550 ℃ causes substrate temper‑softening, bulk hardness of H13 mold drops sharply for LPDC casting mold.
Surface preparation before nitriding: polishing texture, oil residue, rust and oxidation scale must be fully removed. Local oil contamination creates non‑nitriding blind spot, which becomes initial aluminum‑sticking position for aluminum casting mold.
Common failure 1: nitriding‑layer peeling. Main triggers: over‑thick brittle compound‑layer; substrate insufficient hardness; surface residual machining stress; severe thermal‑shock in production for EV structural‑part batches.
Common failure 2: uneven nitriding depth. Caused by poor gas flow inside nitriding furnace; deep‑recess cavity location obtains insufficient nitriding depth; sticking defect preferentially occurs at these positions for CPC counter‑pressure casting mold.
Post‑nitriding operation prohibition: heavy grinding or strong polishing removes protective nitriding layer. Minor finishing is permitted; strictly avoid processing depth exceeding 0.03 mm on functional cavity surface for gravity casting mold maintenance.
Local re‑nitriding restriction: full stripping of old nitriding layer is required before repair nitriding. Residual old nitriding layer prevents new nitrogen atom diffusion and leads to poor bonding between new‑old layer for LPDC casting mold repair.
On‑site acceptance items: total nitriding depth, compound‑layer thickness, surface hardness, metallographic inspection of layer structure. Only relying on surface hardness test cannot fully judge nitriding quality for aluminum casting project.
Cross‑border mold procurement reminder: clearly define nitriding metallographic specification in technical drawing. Many overseas suppliers only guarantee surface hardness index, ignoring compound‑layer thickness control and causing early sticking‑peeling failure.
FAQ
Q: What are two sub‑layers of complete nitriding layer on H13 mold steel?
A: Surface compound‑layer and underneath diffusion‑layer.
Q: What target total nitriding depth range for aluminum casting H13 mold cavity?
A: Total nitriding depth 0.12‑0.20 mm; compound‑layer 5‑12 μm.
Q: What negative consequence arises when nitriding temperature exceeds 550 ℃ for H13 steel?
A: Mold substrate undergoes temper softening, bulk hardness decreases significantly.
Q: Why decarburized mold substrate cannot be fixed by nitriding treatment?
A: Nitriding cannot eliminate substrate surface decarburization defect, performance deficit remains.
Q: What major root‑causes induce nitriding‑layer peeling failure in casting service?
A: Over‑thick brittle compound‑layer, low substrate hardness, residual stress, severe thermal shock.
Q: What post‑nitriding surface‑processing restriction shall be followed for mold cavity?
A: Avoid heavy grinding/polishing; finishing removal shall not exceed 0.03 mm.
Q: Why surface hardness test alone is insufficient for nitriding‑layer acceptance?
A: Hardness cannot reflect compound‑layer thickness, bonding status and total diffusion depth.
