This page contains a Flash digital edition of a book.
EFFECT OF RARE EARTH ADDITIONS ON GRAIN REFINEMENT OF PLAIN CARBON STEELS


R. Tuttle


Department of Mechanical Engineering, Saginaw Valley State University, University Center, MI, USA Copyright © 2012 American Foundry Society


Abstract


This paper describes a set of experiments with misch met- al and rare earth silicide additions in 1010 and 1030 steel to determine whether grain refinement occurs. Target rare earth (RE) contents of 0.1 and 0.2% were employed. After melting, the desired RE addition was added dur- ing tapping and then poured into green sand molds. The resulting test plates were then sectioned for tensile and metallographic testing. Yield strength increased for sev- eral of the 1010 and 1030 samples. The increase in yield strength correlated with a reduction in grain size. A dra-


Introduction


Over the past decade, there have been significant advance- ments in the steels produced by steel mills. These new steels provide higher levels of strength through multi-phase micro- structures. Specialized thermomechanical processing routes and alloy additions cause the increased strength in most of these alloys. Steel foundries cannot use thermomechanical processing routes to increase the strength of their steels. As a result, the advanced steels developed by the wrought steel industry have not been as successful for foundries. The net result is that the steel foundry industry must develop meth- ods of strengthening steel that are effective in their manu- facturing process.


One possible route for improving the strength of cast steels is to reduce the grain size. Decreasing the grain size increas- es the number of grain boundaries which impedes disloca- tion motion within the metal crystal. Decreasing the grain size during solidification requires the introduction of a sig- nificant number of heterogeneous nuclei. A large number of heterogeneous nuclei allow a multitude of steel grains to grow. These more populous steel grains continue to grow in the liquid until they impinge on each other. Since there are more steel grains, impingement occurs sooner than when fewer nuclei are present, providing the resulting reduction in grain size. Grain refinement has been successfully em- ployed in aluminum, magnesium, and copper alloys as a vi- able foundry technology.1-3


The basis of solidification based grain refinement is the ma- nipulation of the number of grains nucleated through hetero-


International Journal of Metalcasting/Spring 2012


matic increase in percent elongation was also observed in the 1010 sample with the smallest grain size. Electron mi- croscopy found complex RE oxides. These oxides appear to act as heterogeneous nuclei. When they were coated with another slag, the grain size and mechanical proper- ties were similar to the baseline material in both the 1010 and 1030.


Keywords: steel, grain refinement, 1010, 1030, rare earth, misch metal, rare earth silicide


geneous nucleation events. This requires the introduction of heterogeneous nuclei into the melt. These nuclei must meet four criteria: be solid at the liquidus temperature of the melt; be thermodynamically stable in the melt; be wetted by the melt; and have a similar crystallographic structure.1,4,5


Refer-


ence data for prospective nucleating phases can readily iden- tify compounds that are solid at the melt’s liquidus tempera- ture. Thermodynamic data can be evaluated by Gibbs’ free energy minimization methods. These determine the stability of a candidate nucleation compound in a melt through com- puterized methods. Wetting data for liquid metal systems is difficult to obtain and frequently unavailable when select- ing candidate phases.2,4


Crystallographic similarity between


the candidate nucleation compound and the solid phase of the melt can be determined using reference crystallographic data and the following equation:


Eqn. 1


where (hkl)s is a low-index plane of the substrate, [uvw]s a low-index direction in (hkl)s the nucleated solid, [uvw]n n , d[uvw]n


bull-Vonnegut equation which provides the ability to deter- mine the average difference between atom positions on the crystallographic planes of two crystal structures.5


interatomic spacing along [uvw]s the [uvw]s and [uvw]n


is the interatomic spacing along [uvw]n


is a low-index direction in (hkl) , d[uvw]s


, (hkl)n . Equation 1 is the Generalized Turn- The result


is a determination of the lattice disregistry, or difference in lattice parameters, between two phases. Experimental evi-


51 is


is a low-index plane in is the


, and θ is the angle between


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  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91