Thermal analysis sTudy of heTerogeneous nuclei in sTainless sTeels R. Tuttle
Saginaw Valley State University, University Center, MI, USA Copyright © 2012 American Foundry Society abstract
This paper presents work done to determine effective heterogeneous nuclei for 304 and HK stainless steels. Heats of each steel were melted in an induction furnace and then poured into a thermal analysis cup which contained the experimental powder. MgO, NbO, NiAl, and TiN powders were added to 304. Powders of ZrO2 La2
,
data acquisition system recorded the cooling curves of the solidifying alloys. Data from these curves was then used to calculate the amount of undercooling to initiate
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Stainless steels are known for their corrosion and oxidation resistance and excellent mechanical properties. This has led to their adoption in a wide variety of industries and applica- tions. The significant amounts of chromium, nickel, and other alloying elements causes these alloys to be more expensive than plain carbon steels. The recent rise in the cost of these elements has impacted their use. To remain competitive, steel mills and foundries producing these alloys are examining ways to reduce cost. Since the chromium and nickel contents for these alloys are required to provide the resistance to en- vironmental conditions that attract users, there are limits to reducing them. Users of wrought stainless steels have ben- efited from improved strength that enables them to reduce the section size of their components. The austenitic stainless steel grades are limited in their strengthening mechanisms. These alloys are not heat treatable due to the stability of austenite and lack the precipitation hardening elements that other stainless grades have. The only option available to austenitic stainless steel producers is to decrease grain size. Steel mills accom- plish this through controlled hot and cold rolling. Cold rolling in particular plays an important role in increasing the strength of stainless steels from steel mills.1 be 20% or more.1
This strength increase can Steel foundries producing stainless steel
castings do not have the ability to mechanically work their material. The result is that stainless steel castings have signifi- cantly lower mechanical properties than their wrought coun- terparts. The lower strength leads to larger section thicknesses than stainless steel weldments of some parts.
Increasing the strength of stainless steels produced via the foundry process requires a different approach. The most like-
International Journal of Metalcasting/Winter 2012 , MgO, and NbO were introduced into HK. A
undercooling in HK. The secondary dendrite arm spacing of the 304 samples were not affected by the addition of the heterogeneous nuclei. It is postulated that this was caused by the high cooling rate of the thermal analysis cup during solidification.
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Keywords: steel, heterogeneous nucleation, grain refinement, thermal analysis, solidification
ly strengthening method is to reduce the grain size through manipulating solidification or inhibiting grain growth. In- hibiting grain growth as the casting cools from the solidi- fication temperature to room temperature requires small in- clusions that pin the grain boundary. It is well known that these inclusions can result in a reduction in elongation and impact properties. Manipulating the solidification of these alloys to reduce grain size provides a better route. Decreas- ing the grain size by manipulating solidification can occur through two routes, cooling rate and improved nucleation. Higher cooling rates produce finer microstructures. Increas- ing cooling can be done by changing the molding process. However, other factors such as cost, production rates, and dimensional requirements usually fix the molding process selected. Even for a given molding process, there are practi- cal limits to the ability to increase the cooling rate of a cast- ing. Improving nucleation would provide a benefit to any molding process. Enabling more grains to nucleate by pro- viding heterogeneous nuclei would decrease the grain size and improve strength. Grain refinement through the applica- tion of heterogeneous nucleation theory has proved viable in aluminum, magnesium, copper, and other alloys.2-5
Heterogeneous nucleation occurs when the growth of the freezing solid initiates due to a foreign particle. The foreign particles reduce the energy barrier necessary for the forma- tion of the solid phase.4
This reduction in energy barrier pri-
marily occurs due to the existing liquid-solid interface of the foreign particle. Without the foreign particle, initiating a sol- id nucleus requires the formation of a liquid-solid interface that requires considerable energy to create. Foreign particles must meet four criteria to be effective heterogeneous nuclei: be solid at the liquidus temperature of the melt; be thermo-
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solidification. MgO, NbO, NiAl, and TiN reduced the undercooling in 304 while La2
, MgO, and NbO reduced
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