Segregation and Banding in Carbon and Alloy Steel
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o one likes surprises in manufacturing, and this is especially true during heat treatment. To avoid them, our attention is often focused on the type of material being supplied to us and the process/equip- ment variables we must control. What we don’t often consider is the condition of the incoming raw material. In this regard, the result of alloy segregation and banding af- ter heat treatment can cause considerable angst between the heat treater and their customer. Let’s learn more. Many people believe steels are classified only by their chemistry. In point of fact, the steelmaking process used; the casting process employed (ingot or continuous casting); the size/shape of semi-finished (blooms, billets or slabs) or finished products (plate, sheet or bars); and the properties they are required to have for the end-use application play a major role. Segregation Steels are not chemically homogeneous; they do not have uni- form chemistry throughout the entire cross- sectional area of their manufactured shapes. When steel is cast, the first material to so- lidify is the outer area adjacent to the mold walls (this surface zone is referred to as the chill zone). This results in a thin layer of equiaxed crystals with the same composition as the liquid metal. These crystals continue to grow inward as columnar grains in a den- dritic shape parallel to the thermal gradi- ent. Finally, solidification ends as the liquid temperature drops and neighboring grains impinge upon each other in the central zone of the as-cast shape (Fig. 1). The solidification process causes macro- scopic as well as microscopic partitioning of chemical elements as the liquid metal cools. Macroscopically, segregation occurs at the centerline of continuously cast products and at the centerline, top and bottom of ingots. Microscopically, segregation occurs between dendrites throughout the solidified section. Subsequent mechanical hot working (e.g., rolling) creates longitudinal bands of varying elemental chemistry. Microsegregation is the difference in composition between the center of the dendritic stem (i.e. the dendritic core) and the region between the dendrite arms. The first liquid to solidify (in the cen- ter of the dendrite) will be alloy-rich if the alloy addition raises the melting temperature, whereas the alloy (i.e. solute-rich) area will be concentrated at the interdendritic regions if the alloy addition lowers the melting temperature. For the heat treater, alloy segregation can produce differences in the hardenability of the steel. The difference in hardenability between alloy-rich and alloy-lean regions can manifest itself by creating harder and softer areas of martensite and mixed transfor- mation products such as bainite. Banding Banding is caused by segregation of the alloying elements during solidification. Subsequent hot-working operations result in segre- gation aligned in the direction of working, which results in the banded appearance delineated in the microstructure. The dis- tribution of microsegregation (in wrought steels) depends on how much working has been done to shape the part. The diffusion rates of the alloying elements in steel con- trol the homogenization of the casting. For example, chromium and molybdenum ho- mogenize readily, while nickel homogenizes very slowly. The alternating bands of vary- ing alloy chemistry result in different micro- structures, orientated parallel to the rolling direction of the material (Fig. 2). Additional mechanical and/or thermal treatments to remove or reduce microsegre- gation add to the cost of manufacture and oftentimes are not considered economically Segregation and Banding in Carbon and Alloy Steel Daniel H. Herring | 630-834-3017 | [email protected] The Heat Treat Doctor N N 16 October 2013 - IndustrialHeating.com In this month's IH Monthly Prescription we talk about some reasons for having a lab. Every month, Dan Herring sits down with IH’s editor, Reed Miller, to talk technical. If you have a topic you would like them to discuss, drop us an e-mail at [email protected]. Find the podcast on our website. IH Monthly Prescription is sponsored by Praxair. Fig. 1. Schematic diagram of zones of crystal morphologies in an as-solidified section of steel. Illustrated are the outer chill zone, the columnar zone and the interior equiaxed zone. Macro and microsegregation are not illustrated in this view. [1]
description
Industrial Heating article on the segregation and banding in steels.
Transcript of Segregation and Banding in Carbon and Alloy Steel
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o one likes surprises in manufacturing, and this is especially true during heat treatment. To avoid them, our attention is often focused on the type of material being supplied to us and the process/equip-
ment variables we must control. What we dont often consider is the condition of the incoming raw material. In this regard, the result of alloy segregation and banding af-ter heat treatment can cause considerable angst between the heat treater and their customer. Lets learn more. Many people believe steels are classifi ed only by their chemistry. In point of fact, the steelmaking process used; the casting
process employed (ingot or continuous casting); the size/shape of semi-fi nished (blooms, billets or slabs) or fi nished products (plate, sheet or bars); and the properties they are required to have for the end-use application play a major role.
SegregationSteels are not chemically homogeneous; they do not have uni-form chemistry throughout the entire cross-sectional area of their manufactured shapes. When steel is cast, the fi rst material to so-lidify is the outer area adjacent to the mold walls (this surface zone is referred to as the chill zone). This results in a thin layer of equiaxed crystals with the same composition as the liquid metal. These crystals continue to grow inward as columnar grains in a den-dritic shape parallel to the thermal gradi-ent. Finally, solidifi cation ends as the liquid temperature drops and neighboring grains impinge upon each other in the central zone of the as-cast shape (Fig. 1). The solidifi cation process causes macro-scopic as well as microscopic partitioning of chemical elements as the liquid metal cools. Macroscopically, segregation occurs at the
centerline of continuously cast products and at the centerline, top and bottom of ingots. Microscopically, segregation occurs between dendrites throughout the solidifi ed section. Subsequent mechanical hot working (e.g., rolling) creates longitudinal bands of varying elemental chemistry. Microsegregation is the difference in composition between the center of the dendritic stem (i.e. the dendritic core) and the region between the dendrite arms. The fi rst liquid to solidify (in the cen-ter of the dendrite) will be alloy-rich if the alloy addition raises the melting temperature, whereas the alloy (i.e. solute-rich) area will be concentrated at the interdendritic regions if the alloy addition lowers the melting temperature. For the heat treater, alloy segregation can produce differences in the hardenability of the steel. The difference in hardenability between alloy-rich and alloy-lean regions can manifest itself by creating harder and softer areas of martensite and mixed transfor-mation products such as bainite.
BandingBanding is caused by segregation of the alloying elements during solidifi cation. Subsequent hot-working operations result in segre-
gation aligned in the direction of working, which results in the banded appearance delineated in the microstructure. The dis-tribution of microsegregation (in wrought steels) depends on how much working has been done to shape the part. The diffusion rates of the alloying elements in steel con-trol the homogenization of the casting. For example, chromium and molybdenum ho-mogenize readily, while nickel homogenizes very slowly. The alternating bands of vary-ing alloy chemistry result in different micro-structures, orientated parallel to the rolling direction of the material (Fig. 2). Additional mechanical and/or thermal treatments to remove or reduce microsegre-gation add to the cost of manufacture and oftentimes are not considered economically
Segregation and Banding in Carbon and Alloy Steel
Daniel H. Herring | 630-834-3017 | [email protected]
The Heat Treat Doctor
NN
16 October 2013 - IndustrialHeating.com
In this month's IH Monthly Prescription we talk about some reasons for having a lab. Every month, Dan Herring sits down with IHs editor, Reed Miller, to talk technical. If you have a topic you would like them to discuss, drop us an e-mail at [email protected]. Find the podcast on our website. IH Monthly Prescription is sponsored by Praxair.
Fig. 1. Schematic diagram of zones of crystal morphologies in an as-solidifi ed section of steel. Illustrated are the outer chill zone, the columnar zone and the interior equiaxed zone. Macro and microsegregation are not illustrated in this view.[1]
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feasible. For example, a high degree of ho-mogenization can result from soaking the segregated steel at an elevated tempera-ture (often above 1200C or 2200F) for a very long time (up to 100 hours or more in some cases). The primary cause of banding is due to the segregation of substitutional alloying elements (e.g., manganese, chromium, molybdenum) during (dendritic) solidi-fi cation. Cooling rate, austenitic grain size and austenitizing temperature also infl uence the severity of microstructural banding. Banding occurs in all steels. While reheating of as-cast products and hotrolling tend to reduce chemical segregation, other factors (related to solid-state phase transformations and residual solidifi cation) result in a greater or lower degree of banding in the microstructure of all fi nished steel products.
Effect of Banding on Heat TreatmentIn general, hardness and microstructure will be heavily infl uenced by segregation and banding. Alloy-rich areas tend to transform to martensite or bainite, while alloy-lean areas show increasing amounts of pearlite and ferrite (due to slower cool-
ing rates). For case-hardened parts, the hardness of the case will be impacted, particularly if higher concentrations of retained austenite or bainite are formed in the primarily martensite structure. In certain applications, retained austenite converts to untempered martensite in service (Fig. 3), resulting in hardness and property variation that can cause dimen-sional change or even component failure. Tensile, yield and fatigue strength tend to be relatively unaffected by the presence or absence of microsegregation, although ductility and toughness properties are neg-atively impacted. As an example, spotty hardness and poor mechanical properties can result from segregated or banded microstruc-tures having signifi cant amounts of coarse pearlite, coarse ferrite or ferrite clusters. In induction hardening, higher temperatures and longer heating times are required to fully austenitize these struc-tures and may result in grain growth, for-mation of coarse martensite on quench-ing, surface oxidation/decarburization and increased chance of distortion. Thus, having a prior microstructure consisting of fi ne pearlite or a quench-and-tempered structure is the most highly desired for induction hardening.
SummarySegregation and banding may or may not be detrimental to the fi nal product, but the effects of these conditions should either be evaluated before the raw material is heat treated or negated (to the extent possible) by appropriate thermal treatments (e.g., annealing, normalizing). IH
References1. Krauss, George, Solidifi cation, Segregation
and Banding in Carbon and Alloy Steels, Metallurgical and Material Transactions B, Volume 34B, December 2003, pp. 781 792.
2. Majka, Ted F., David K. Matlock and George Krauss, Development of Microstructural Banding in Low-Alloy Steel with Simulated Mn Segregation, Metallurgical and Materials Transactions A, Volume 33A, June 2002, pp. 1627 - 1637.
3. Shewman, Paul G., Transformations in Met-als, McGraw-Hill, Book Company, 1969.
4. Parrish, Geoffrey, Carburizing: Microstruc-ture and Properties, ASM International, 1999.
5. VanAken, David, Engineering Concepts: Segregation and Banding in Steel, Indus-trial Heating, April 2001.
6. Rudenev, Valery I., Can the Fe-Fe3C Phase Transformation Diagram be Directly Ap-plied in Induction Hardening of Steel?, Heat Treating Progress, June/July 2003.
7. Rudnev, V., D. Loveless, R. Cook and M. Black, Handbook of Induction Heating, Mar-cel Dekker, Inc. 2002.
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Fig. 2. Banded 4340 microstructure (50x, Vilellas Reagent)
Fig. 3. 4340 clutch plate. A region of reformed (untempered) martensite (white) within the banded area of the heat-affected zone. This area exhibited hardness values signifi cantly higher than those of the surrounding area. (1250x, Vilellas Reagent)
