Surface hardening process refers to make surface hardening of investment castings,but the core still have strong toughness.Through this process,we can improve the wear resistance and fatigue resistance of investment castings.As the core still has good toughness and strength,so investment castings after case hardening have good resistance to impact load.Surface hardening treatment methods mainly include carburizing&quenching, nitriding, hard anodic oxidation, chrome plated, and metallic cementation, etc.
Refers to heating alloy steel investment castings(generally contain Al, Cr, Mo) in anhydrous ammonia (NH3) flow at about 500 ℃, 570 ℃ for a long time.The surface of steel surface will form a layer of high hardness and corrosion resistance of nitride (mainly Fe2N Fe3N Fe4N).Generally speaking,there are gas nitriding,liquid nitriding and glow ion nitriding.
3)Hard anodic oxidation
Hard anodic oxidation is a case hardening way for aluminum die castings.Anodic oxidation is to use aluminum or aluminum alloy as anode and use lead plate as cathode,then electrolyze in the electrolyte and produce oxidation film layer on the surface.After anodic oxidation, aluminum surface can generate the oxide film thickness for several to several hundred microns.This surface of the oxide film layer is porous honeycomb, compared with the natural oxidation film of aluminum alloy, its corrosion resistance,abrasion resistance and decorative has obviously improved.Using different electrolyte and process conditions, we can get different properties of anodic oxide film.
The hardness of Chromium can reach 800 ~ 1000HV, is one of the most hard metal.It has a very strong passivation performance, in the atmosphere quickly passivate, thus chromium layer in the atmosphere is very stable, could maintain its luster in the long term, and organic acid corrosion medium is very stable like the alkali, nitric acid, sulfide, carbonate.But soluble in halogen acid,like hydrochloric acid and hot concentrated sulfuric acid.