| Summary: | 博士 === 大同大學 === 材料工程研究所 === 88 === Compacted/vermicular graphite cast iron gradually becomes an important material of cast iron. Which has greatly mechanic and physics properties. Austempering heat treatment can make basic materials become Ausferrite which has high strength、toughness、wear resistant and anti-fatigue characteristics. This experiment nodularizer’s add amount to controlled molten iron so as to produce the Compacted/vermicular graphite cast iron. Meanwhile it looks giving the positives fracture toughness of as-cast after it has been disposed of by Austempering heat treatment.
After different dispose of the Austempering heat treatment, it separately produces the upper and lower ausferrite and the mixed structure of upper and lower ausferrite. The result shows that the mixed structure of the upper/lower ausferrite has high strength and the upper ausferrite comes after. The lowest strength is the lower ausferrite.
The results showed that when spheroidizer applied to molten iron in the amount of 0.9wt% CG iron of over 90% vermicularity was produced without copper or titanium alloying. But CG iron of over 95% vermicularity was produced with copper or titanium alloying, because copper or titanium alloying seemed to refine graphite morphology, promote the formation of pearlite and restrain the percentage of nodularity. Besides the different thin specimens without copper or titanium addition yielded better fracture toughness than the specimen with copper or titanium addition, except the 30mm thick specimen. The might attributed to the lower vermicularity and higher mechanical properties of the thinner castings.
Austempering heat treatment was applied to gray cast iron in order to study the effect of matrix toughening on fracture mechanics behavior of it. The material was austenitized at 900℃ for 1.5hours and then austempered at 300℃/3hours or 360℃/2hours so as to obtain different matrix morphology, lower ausferrite or upper ausferrite. A mixed matrix structure was also achieved by 300℃for 172 minutes. The results showed that, quite surprisingly and contrary to the general belief that flake graphite dominate and control all properties of gay iron, the plane stress fracture toughness of the cast iron. So treated were increase from the as-cast of 12.3 MPa to that of 16.0 MPa , 23.8 and 26.1 MPa respectively for the above heat-treated conditions. These were marked improvement of 30%, 93%, and 112% for the flake graphite cast iron with upper-ausferrite , lower-ausferrite, and mixed upper/lower ausferrite matrices.
The very little plastic deformation exhibited in the fracturing of flake graphite (FG) cast irons and compacted graphite (CG) cast irons qualified them as brittle materials. The fatigue precracking of these brittle material in KIC plane strain fracture toughness testing is a difficult task. Opposed to this static toughness KIC, the dynamic toughness obtained by impact testing does not have such bottleneck. Since the stress-strain behavior of brittle material is essentially linear, and the fracture appearance of the specimens after impact testing must largely be flat without shear-lip, these conditions also satisfy the linear-elastic requirement in KIC testing. Thus, it is of interest to find out if there’s any linear relationship existed between these two toughness properties of brittle material. Annealing, normalizing, and austempering heat treatment were applied to FG and CG irons to alter their matrix structures so as to obtain a range of static (KIC) and dynamic (impact) toughness values. A third toughness, calculated by integrating the area under the load vs. CGD (Crack Gage Displacement) curve in KIC testing, was termed “ Calculated toughness” and was also listed for comparison purpose. It was found among static toughness KIC(Y), dynamic impact toughness(X), and calculated toughness(Z), linear relationships exist for flake graphite cast iron (FG) and compacted graphite cast iron (CG) as：
FG：Y=4.02X－0.81
CG：Y=0.86X＋20.2
FG：Z=1.75X＋2.99
CG：Z=0.77X＋4.77
Compacted cast irons have agreed on specifications (CNS). Compacted cast irons are still very strange in industry. Therefore，how to make use of in any kinds of machines、tools and molds are a goal in materials in future. Consequently，how to popularize vigorous develop in science and technology nowadays is one of an essentially lessons. Molds are particular about Q.D.C. now. Compacted cast irons can be make use of thickness section and addition Cu and Ti alloy element ,etc. It has high mechanical properties and conducts as die base，will be a great break through in mold.
|
|---|
