The Investigation On Spot Array Laser Alloying On Nodular Cast Iron Hot Roller

According to the urgent demands on extending the working life of nodular cast iron hot rollers, and the severe crack problem on conventional large-area laser alloying technology, this thesis reports systematic experimental study on a novel spot-array laser alloying, that adopts the discrete strengthening idea, on three different kind of nodular cast iron samples. The contrast thermal fatigue tests were performed on laser treated nodular cast iron samples and untreated nodular cast iron samples. The thermal fatigue crack and microstructure of the samples were analyzed before and after tests. A laser chopper was designed to out put pulse laser.The experimental results indicate that spot-array laser alloying on chilled nodular cast iron sample leads to crack across the whole alloyed layer. The spot-array laser alloying on pearlitic nodular cast iron sample, and quenched/tempered nodular cast iron sample can result in a good surface smoothness that is free of cracks under appropriate processing parameters.The research demonstrates that the microstructure of alloyed layer consists of eutectic structure or hypereutectic structure close to eutectic, the microstructure in heat affected zone transformed to residue austenite, martensite and refined pearlite due to the different cooling speeds. The maximum microhardness in alloyed zone is V0.2900, and the microhardness in heat affected zone is also much higher than the substrate.The thermal fatigue tests indicate that the quenched/tempered nodular cast iron sample has the best thermal fatigue resistance, cracks occur in alloyed zone after tests, but these cracks can not cross heat affected zone and could not propagate in to substrate; a large number of resillage occur on the surface of untreated pearlitic nodular cast iron sample after tests, while the number and density of cracks on the surface of laser treated pearlitic nodular cast iron samples were much lower than the untreated samples. Cracks also occur in the alloyed zone, but the cracks within alloyed zone can not connect the crack in the substrate because of the retarding effect of heat affected zone. The investigation results indicate that the retarding effect of heat affected zone is attributed to its particular microstructure. The thermal fatigue crack was forced to propagate alone with the grain boundary as a zigzag way due to the good ductility of residue austenite, lath martensite and refining pearlite, the good strength of lath martensite and refining pearlite, and the compressive stress of martensite. This kind of propagation greatly consumed the propagation energy of crack.