| High Ni-Cr indefinite chill double-poured (ICDP) is a kind of excellent thermal rollmaterials. The composite rolls produced by centrifugal casting method are widelyrecognized as the first choice for the later stand of hot rolling mill due to their superiorthermostability, good resistance to adherence and thermo-cracking. However, as the highspeed steel and high-Cr cast iron roll are widely used in the former stand of hot mill rack,it will greatly extend its use cycle. So that it requires higher qualities of the high Ni-Crcomposite cast iron rolls for the later stand. Therefore, improving the microstructure andproperties of high Ni-Cr composite cast iron rolls is of significance to enhance the rollsquality, prolong the rolls life, improve the mill working efficiency and reduce steel rollingproduction costs.In this paper, the ladder pouring method was used to obtain the high Ni-Cr compositecast iron. The effects of75SiFe inoculation, REMg modification and V&Ti alloying onmicrostructure and hardness of high Ni-Cr cast iron were studied by the way ofmetallographic microscope, SEM, EDS, XRD and hardness measurement, et al. And theinfluence of high Ni-Cr cast iron microstructure on its high temperature wear resistanceand thermal fatigue properties was also investigated. Meanwhile, the mechanisms of hightemperature wear and thermal fatigue crack were discussed. The results are shown asfollows:The influences on high Ni-Cr cast iron microstructure and hardness after75SiFeinoculation and REMg modification are comparatively studied, and the result shows that75SiFe inoculation promotes flake graphite precipitation and the graphite contentsincrease from4.2%to8.1%; REMg modification promotes spheroidal graphiteprecipitation and the contents decrease from4.2%to2.4%. The continuous net-likestructures of carbides are broken partially and change into island-like and bone-likestructures, REMg modification can also enhance the hardness, but high chilling speed willweaken the modification effects. When0.3%V and Ti are added in the ordinary high Ni-Cr cast iron, eutectic carbideand graphite grain sizes are refined and VC, TiC, et al carbides are formed, which willenhance its high temperature wear resistance. However, V&Ti alloying will increase thecontents of retained austenite in the as-cast microstructure of high Ni-Cr cast iron, makingthe as-cast materials hardness decreasing slightly.Many retained austenite exist in75SiFe inoculated and REMg modified high Ni-Crcast iron. Even at350oC tempering conditions, the retained austenite also exist stably.With the increase of tempering temperature and time, the contents of the retained austenitedecrease and the bainite will replace retained austenite gradually. While75SiFe inoculatedhigh Ni-Cr cast iron hardness will decline slightly with the increase of temperingtemperature and time; REMg modified high Ni-Cr cast iron shows secondary hardening at400oC×20h tempering, its peak hardness is62.3HRC. when the tempering temperaturereaches to500oC, retained austenite will change to tempered sorbite, also the temperinghardness decreases.In the test of high temperature wear at550oC,75SiFe inoculated and REMg modifiedhigh Ni-Cr cast iron abrasive wear weight loss will increase along with the load. Under thesame chosen load, abrasive wear weight loss of75SiFe inoculated high Ni-Cr cast iron islower than that of REMg modified high Ni-Cr cast iron, demonstrating its good hightemperature wear resistance. The mechanism of high Ni-Cr cast iron is motivated byadhesive oxidation and touching fatigue wear.After20times tests of thermal fatigue for high Ni-Cr cast iron at temperature rangingfrom700to20oC, the crack lengths are15.49mm and17.69mm respectively for75SiFeinoculated and REMg modified samples. The graphite in75SiFe inoculated samples istiny and flake, its thermal fatigue crack starts at graphite tip, extending circuitously alongthe interface of austenite and ledeburite. While the crack in REMg modified samples isgenerated inside the carbides, extending straightly along the carbides. |
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