EXAMINATION OF STEEL CASTINGS FOR POTENTIAL NUCLEATION PHASES R. Tuttle
Saginaw Valley State University, University Center, MI, USA Copyright © 2010 American Foundry Society Abstract
The production of steel castings needs to meet more stringent requirements on mechanical properties and cost. One pos- sibility to improve mechanical properties is to develop grain refiners for steel castings. There are no industrially significant grain refiners for steels. While developing grain refiners based on theoretical models could assist development, there is likely
Introduction
The steel foundry industry could improve mechanical prop- erties and lower costs to meet the increasingly stringent de- mands of customers and competition from other alloys. To improve fuel economy and reduce cost, vehicle manufactur- ers are examining reduction in section thicknesses or lower density metals. Lower density metals can require larger sec- tion thicknesses to handle the loading on a component. Thin- ner steel castings can provide reduced weight through sec- tion size reduction; however, the steel alloys employed must have higher strength than current alloys. Ductile iron alloys have penetrated steel casting applications due to their lower cost and density. Stronger steel alloys with lower alloying element content could provide an attractive alternative to ductile iron. Steel foundries also are experiencing unprec- edented price increases for alloying elements. The develop- ment of an effective grain refiner for steel alloys is a possible route for improving mechanical properties in steels with a small alloying addition.
Grain refinement is a metallurgical technique common in nonferrous alloys. Aluminum alloys are a commonly grain refined nonferrous alloy. Researchers and industrial opera- tors report improved strength and ductility in these alloys.1,2,3 Aluminum experimenters have also observed reduced segre- gation and hot tearing when these alloys are grain refined.2,3,4 Despite these observed improvements, grain refinement has not been applied to steel alloys.
Bramfitt was amongst the earliest researchers to work on grain refinement in ferrous alloys.5
He conducted a series
of experiments utilizing a vacuum-induction furnace and high purity iron. The high purity iron was melted in a he- lium atmosphere. Once molten, an alumina rod with -325 mesh powder bonded to it with sodium silicate binder was
International Journal of Metalcasting/Summer 10
already materials that function in current castings as nuclei for austenite dendrites. It was found that TiN inclusions may have assisted in the nucleation of dendrites during the solidification of several industrial castings.
Keywords: steel, grain refiner, examination
immersed in the liquid. The melt was allowed to cool, and an S-type thermocouple recorded the cooling curve. Bramfitt evaluated the effectiveness of various powders for nucleat- ing the steel from the amount of undercooling necessary for initiating solidification. TiN, TiC, and SiC were found to re- duce the degree of undercooling the most. Bramfitt verified his findings by calculating the lattice disregistry between the crystal structure of these phases and δ-ferrite. The phases with the smallest disregistry produced refinement.
J.F. Wallace and coworkers conducted a series of laboratory experiments that more accurately reflected industrial prac- tice.6
The experiments consisted of induction melting 1018 steel and adding powders of FeNb, FeTi, FeB, Zr, TiC, FeV, W, AlCo, AlNi, and ZrC to the melt. The treated metal was poured into a mold. Then, the samples were metallographi- cally examined for grain refinement. Niobium, titanium, and titanium carbide were found to produce small equiaxed grains. Initially, Wallace et al. assumed that these phases produced refinement by limiting grain growth.6
This conclu-
Titanium carbides were successfully formed, and grain refinement was observed. Tensile testing was done to determine how the mechanical properties were affected by grain refinement. Wallace et al. found that yield strength and ultimate tensile strength both increased; however, reduction in area dramatically dropped. The authors attributed the re- duction to the TiC particles that were at the grain boundaries. Several different heat treatments were attempted to improve the reduction in area of the treated steels. Unfortunately, the heat treatments necessary to obtain an improved reduction in
In later work, Wallace attempted to form titanium carbides by adding additional titanium to a 1018 induction melted steel.8
sion was drawn from of the presence of TiC or NbC particles around the ferrite grains. However, later publications by Wal- lace incorporated the conclusions from Bramfitt’s work and believed that TiC nucleated δ-ferrite causing the observed re- finement.7
17
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81