STUDY OF PREMIUM H13 FOR

IMPROVED TOUGHNESS AND TEMPERATURE RESISTANCE

Final Report

Roger N. Wright Materials Science & Engineering Department

Rensselaer Polytechnic Institute Troy, NY

January 10, 2003

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EXECUTIVE SUMMARY

The Forging Industry Educational & Research Foundation (FIERF) awarded Rensselaer Polytechnic Institute a FIERF/FINKL CHALLENGE GRANT, and the grant program was entitled “Study of Premium H13 for Improved Toughness and Temperature Resistance”. The objectives of the program were to characterize selected H13 microstructures with emphasis on size range and homogeneity of distribution of carbides, and to examine relationship of the microstructure to Charpy V-Notch impact toughness, and to the details of the fracture path. This document is the Final Report for the grant program. A rigorous metallographic examination of carbide size distribution has been undertaken for three conventionally processed H13 heats, and a modified H13 composition with lower silicon, lower vanadium, and higher molybdenum. One of the conventionally processed H13 heats displayed Charpy V-Notch impact values below the acceptance criterion of NADCA Recommended Procedure #207-97. This same material contained markedly larger carbide size distributions than displayed by the three other, acceptably tough materials. The fractographic characteristics of this low toughness material revealed that pores developing from coarser carbides pre-empt the normal fracture mechanism. Application of a classical fracture model indicates that the reduced fracture strain/toughness observed for the low toughness material corresponds to an order-of-magnitude increase in the population of carbides contributing to fracture. Scanning electron microscope and electron beam microprobe studies have indicated that the coarser carbide structure is composed of molybdenum and vanadium carbides, and a technique has been employed that will allow the relationships amongst carbide type (molybdenum, vanadium, etc.), carbide size/shape and impact toughness to be detailed.

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ACKNOWLEDGMENTS The diligent and painstaking metallographic, electron microscopic and microprobe analysis work undertaken by Rensselaer Undergraduate Research Participation Program members David Giuliano and Jason Kane is gratefully acknowledged, as is the work of their mentor, Robert Planty, manager of the Rensselaer Materials Analysis Laboratories. Of course, the project would not have been possible without the FIERF/FINKL CHALLENGE GRANT and the guidance and research contributions of John Paules of Ellwood Materials Technologies.

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TABLE OF CONTENTS

Section Page Executive Summary…………………………………………………………. 2 Acknowledgments…………………………………………………………… 3 Introduction…………………………………………………………………. 5 Background…………………………………………………………………. 6 Microstructural Characterization and Relationship of Microstructure to Impact Toughness………………………………………………………… 7 Relationship of Microstructure to Fracture Path…………………………….. 11 Future Work…………………………………………………………………. 15 Summary and Conclusions………………………………………………….. 16 Appendix……………………………………………………………………. 17

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INTRODUCTION On October 29, 2001, Rensselaer Polytechnic Institute submitted a Grant Request Application to the Forging Industry Educational & Research Foundation (FIERF), resulting in FIERF’s February 5, 2002 award of $10,000 to Rensselaer in the form of a FIERF/FINKL CHALLENGE GRANT. The grant program is entitled “Study of Premium H13 for Improved Toughness and Temperature Resistance”. The objectives of the program can be summarized as follows.

1. Characterize selected H13 microstructures, with emphasis on size range and homogeneity of distribution of carbides.

2. Examine relationship of microstructure to Charpy V-Notch (CVN) impact toughness.

3. Examine relationship of microstructure to the fracture path.

The grant program has been completed, and the objectives have been met. This document is the Final Report for the grant program.

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BACKGROUND

The die casting industry makes wide use of H13 as a die steel. The compositional range of this classical alloy is as follows:

C Mn Si Cr Mo V____ 0.32-0.45 0.20-0.50 0.80-1.20 4.75-5.50 1.10-1.75 0.80-1.20 The alloy is produced by various methods (conventional, ESR, VAR, etc.) en route to a quenched and tempered condition with a hardness in the range of Rc 42 and a CVN impact toughness of at least eight foot-pounds. It has become a common practice for die casting customers to specify “premium” grades of H13, possessing elevated CVN impact toughness, and presumably improved service performance (resistance to thermal fatigue, etc.). It has been suggested that superior toughness is associated with minimization of large (20-50 µm), primary chromium and vanadium carbides, and that processes leading to their minimization will produce “premium” quality H13. Nonetheless, considerable variation in H13 toughness is to be noted, and conventional H13 processing is capable, in some instances, of producing H13 with elevated toughness. In this context, Rensselaer proposed undertaking a rigorous characterization of selected H13 microstructures, with emphasis on size range and homogeneity of distribution of carbides. In particular, fractured CVN specimens could be characterized regarding the nature of carbides, the relationship of such carbides to impact toughness, and the relationship of the microstructure to the fracture path. The program addresses the forging industry need for an understanding of the microstructural basis of superior die steel toughness. The results of this work should allow a sophisticated re-examination of the requirements for the achievement of H13 die materials with elevated toughness. The work could be undertaken with the collaboration of Ellwood Materials Technologies Company, as discussed in detail with John R. Paules, General Manager.

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MICROSTRUCTURAL CHARACTERIZATION AND RELATIONSHIP TO IMPACT TOUGHNESS

The bulk of the microstructural characterization work on the grant project has been set forth in detail in a recent publication (R. N. Wright, D. M. Giuliano, J. M. Kane and J. Paules,“Structure/Property Variations in H13 Tool Steels”, Proceedings, 24th Forging Industry Technical Conference, Cleveland, OH, October 14-16, 2002, Forging Industry Educational and Research Foundation, Cleveland, OH). A copy of this publication appears in the APPENDIX of this grant project Final Report. As reported in this appended publication, a rigorous metallographic examination of carbide size distribution was undertaken for three conventionally processed H13 heats, and a modified H13 composition with lower silicon, lower vanadium, and higher molybdenum. One of the conventionally processed H13 heats displayed CVN impact test values below the acceptance criterion of NADCA Recommended Procedure #207-97. This same material contained markedly larger carbide size distributions than displayed by the three other, acceptably tough materials. Specifically, the low toughness heat Heat A (in the published work) had a average CVN impact test value of 7 foot-pounds, in comparison to an average of 17, 17 and 19, respectively, for Heats B, C, and D. Carbide sizes above 1 square micron (:m) were analyzed, and Heat A displayed average and maximum carbide sizes of 4.1 and 92.3 square microns, respectively. In contrast, the average carbides sizes for Heats B, C, and D were 1.8, 1.5 and 1.6 square microns, respectively, and the maximum carbide sizes for Heats B, C, and D were 20.7, 3.2 and 4.8 square microns, respectively. It must be emphasized that the carbide sizes of Heat A are not as large as those of the most inferior product forms, and the data clearly suggest important effects of more subtle carbide size variations. The relatively coarse carbides and low impact properties of Heat A are believed to be related to that material receiving a different homogenization treatment than the others. The carbide size characterization summarized above involved visible light microscopy and image analysis software. Further characterization was undertaken with a scanning electron microscope (SEM) and wavelength dispersive spectroscopy (WDS). Backscattered imaging was carried out on a polished sample from Heat A. Backscattered imaging allows the identification of atomic species with different atomic weights. One result is that higher atomic weight species manifest a brighter image, and lower atomic weight species appear darker. This response is illustrated in Figure 1 of this Final Report (not to be confused with Figure 1 of the publication in the Appendix), where the carbides appear either as bright spots or dark spots. WDS analysis confirmed that the bright spots contained molybdenum (atomic weight 95.95), but was not able to

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confirm the identity of the dark spots, which could conceivably contain vanadium (atomic weight 50.95) or chromium (atomic weight 52.01). It was also confirmed that some of the dark spots were simply areas where the carbide had “pulled out” during sample preparation.

Figure 1. SEM backscattered image of Heat A, displaying carbides as bright and dark spots. WDS analysis confirmed that the bright spots contained molybdenum. Electron beam microprobe (EBM) analysis was used to obtain greater discrimination amongst the carbides. As shown in Figures 2 and 3 of this Final Report, the concentration mapping makes it clear that the brighter spots contain molybdenum and the darker spots contain vanadium. Chromium and iron are rather evenly dispersed throughout the structure and, thus, the coarser carbides can not be chromium or iron carbide. Thus it appears that EBM analysis will allow a further break down of carbide size distribution for the specific species of molybdenum and vanadium carbide. Such an analysis is beyond the scope of the current grant project, but is an obvious target for future work, as discussed below.

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Figure 2. EBM concentration maps displaying a general backscattered image (BE4-map), with bright and dark carbides, and maps indicating the presence of molybdenum (Mo 4-map) and vanadium (V 4-map).

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Figure 3. EBM concentration maps displaying a general backscattered image (BE4-map), with bright and dark carbides, and maps indicating the rather even distribution of chromium (Cr 4-map) and iron (Fe 4-map).

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RELATIONSHIP OF MICROSTRUCTURE TO FRACTURE PATH

The fracture path was initially evaluated by rigorous SEM characterization of Heats A and C. The higher toughness Heat C generally displayed a fracture surface often described as “quasi-cleavage”, as shown in Figure 4. Such a fracture surface is typical of many tempered martensitic structures. It would appear that fracture occurs in a cleavage-like manner, on a scale consistent with the fineness of the acicular martensitic structure. Such structures can manifest significant toughness, and the use of the term quasi-cleavage is not necessarily meant to imply brittleness. Heat C, in fact, had an average CVN impact toughness of 17 foot-pounds. The lower toughness Heat A displayed a rather different fracture surface, with regions of quasi-cleavage, but with a large amount of dimpled rupture, as shown in Figure 5. Figure 5 reveals that many of the dimples still retained carbides at their centers. While dimpled rupture is normally associated with a ductile fracture mode, dimples, themselves, reduce ductility. The basic concept is that weak points in the microstructure, or “fracture centers”, rupture and produce pores, as strain ensues. These pores eventually link up and produce a dimpled fracture surface, with some dimples containing evidence of the source of microstructural weakness (carbides in the present case). Larger numbers of fracture centers and dimples are associated with reduced ductility. A simple theoretical model for dimpled rupture has been summarized by Backofen (W. A. Backofen, Deformation Processing, Addison Wesley Publishing Company, Reading, MA, 1972, pp. 247-250). Figure 6 shows a schematic basis for such a model. On the left-hand side of Figure 6, fracture centers have opened and grown to a nominal width, w, with a center-to-center spacing of l. The overall cross sectional area, Ao , is approximately nl

2, where n is the number of fracture centers. The fracture center volume is about nw2, and the fracture center volume fraction, ∆∆ , is thus roughly (nw2/nl2) or (w/l)2. On the right-hand side of Figure 6, final fracture is represented, with the areas between pores having undergone “pure rupture” by thinning down to a point. The area at fracture, Af , is approximately nw2. Now the fracture strain can be represented by ln(Ao /Af ) which equals ln(nl2/nw2) or ln(1/∆∆). In application to the present analysis, the relatively large carbides of Heat A appear to contribute fracture centers and thereby reduce the fracture strain and toughness. In an effort to relate fracture strain to CVN impact toughness and the model presented above, the percent lateral contraction at the base of the CVN specimen notch was measured on the fracture surfaces of the CVN specimens of Heats A and C. Figure 7 shows a CVN specimen fracture surface with the point of maximum lateral contraction indicated by arrows. Heats A and C displayed 4.3% and 6.8% maximum lateral contractions, respectively. Treating these data as fracture strains, equal to ln(1/∆∆), one can infer a thirteen-fold increase of fracture center volume (large carbides) for Heat A, over Heat C.

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Figure 4. Typical “quasi-cleavage” fracture surface appearance of Heat C, with higher CVN impact toughness values. Figure 5. Typical fracture surface appearance of much of Heat A, with low CVN impact toughness values. Large, carbide-related dimples are present.

5::m

5::m

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Figure 6. Relevant ductile fracture model, from Backofen, whereby fracture center volume fraction is ∆∆ = (w/l)2, and the fracture strain is roughly ln(1/∆∆).

Figure 7. Macrofractograph of CVN specimen showing fracture strain manifested by lateral contraction.

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Using fracture strain models, such as presented above, carbide-based fracture centers can, to a first approximation, be related to low-cycle and thermal fatigue lifetimes, since such lifetimes are functions of fracture strain, via Coffin-Manson theory (L. F. Coffin, “Fatigue in Machines and Structures – Power Generation”, Fatigue and Microstructure, American Society for Metals, Metals Park, OH, 1979, pp. 1-27; S.S. Manson, Thermal Stress and Low cycle Fatigue, McGraw-Hill Book Company, New York, 1960.). That is, increased carbide volume can be theoretically related to diminished thermal fatigue resistance. In a further effort to determine the role of carbides with respect to the fracture path, longitudinal metallography was performed on CVN specimens from Heats A, B, C and D, with the polishing plane being perpendicular to and intersecting with the fracture surface. Visible light micrographs are shown in Figures 8 and 9, for Heats C and A, respectively. While the views are different and there is evidence of apparent dimple profiles in Heat A, this technique was judged to be of limited value.

Figure 8. Profile of fracture path in CVN specimen from higher toughness Heat C.

Figure 9. Profile of fracture path in CVN specimen from lower toughness Heat A.

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FUTURE WORK

Concepts for future work include the following:

• The relationships amongst carbide type (molybdenum, vanadium, etc.), carbide size/shape and impact toughness should be detailed, utilizing the mapping and software analytical capabilities demonstrated in the current study.

• Microanalysis and carbide size/shape characterizations should be

undertaken on CVN specimen fracture surfaces to determine the nature of carbide populations that act as fracture centers.

• Additional H13 heats should be examined to assess the generality of the observations set forth in the current study.

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SUMMARY AND CONCLUSIONS A rigorous metallographic examination of carbide size distribution has been undertaken for three conventionally processed H13 heats, and a modified H13 composition with lower silicon, lower vanadium, and higher molybdenum. The implications regarding impact toughness and fracture path have been evaluated. One of the conventionally processed H13 heats displayed Charpy V-Notch values below the acceptance criterion of NADCA Recommended Procedure #207-97. This same material contained markedly larger carbide size distributions than displayed by the three other, acceptably tough materials. The fractographic characteristics of this low toughness material revealed that the normal “quasi-cleavage” fracture mechanism is pre-empted by the growth of dimples from carbide-related fracture centers. Fracture strains in Charpy V-Notch specimens have been determined from lateral contractions at the notch tip. Application of a classical model for dimpled rupture indicates that the reduced fracture strain/toughness observed for the low toughness material corresponds to an order-of-magnitude increase in the population of carbides acting as fracture centers. Scanning electron microscope and electron beam microprobe studies have indicated that the coarser carbide structure is composed of molybdenum and vanadium carbides, and a technique has been employed that will allow the relationships amongst carbide type (molybdenum, vanadium, etc.), carbide size/shape and impact toughness to be detailed.

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APPENDIX STRUCTURE/PROPERTY VARIATIONS IN H13 TOOL

STEELS

Roger N. Wright, David M. Guiliano and Jason M. Kane Rensselaer Polytechnic Institute, Troy, NY

John Paules, Ellwood Materials Technologies, Ellwood City, PA

ABSTRACT

A rigorous metallographic examination of carbide size distribution has been undertaken for four H13 materials, namely three conventionally processed H13 heats, and a modified H13 composition with lower silicon, lower vanadium, and higher molybdenum. One of the conventionally processed H13 heats displayed Charpy V-Notch values below the acceptance criterion of NADCA Recommended Procedure #207-97, whereas the other three heats exhibited toughness above the acceptance criterion. The low toughness heat contained markedly larger carbide size distributions than displayed by the three other, acceptably tough materials.

BACKGROUND Rensselaer Polytechnic Institute submitted a grant request to the Finkl Challenge Program of the Forging Industry Education and Research Foundation on October 31, 2001, and the request was accepted February 5, 2002. The project is entitled “Study of Premium H-13 for Improved Toughness and Temperature Resistance”, and is being undertaken with the collaboration of Ellwood Materials Technologies. AISI H13 is a classic chromium hot work tool steel, in wide use for die casting dies. Its compositional ranges, in weight percent, are as follows.

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_ _ C _ _ Mn _ ___Si___ ___Cr__ ___Ni___ ___Mo__ ___V ___ 0.32-0.45 0.20-0.50 0.80-1.20 4.75-5.50 0.30 max 1.10-1.75 0.80-1.20 It has become a common practice for die casting customers to specify “premium” grades of H13, such as prescribed in NADCA Recommended Procedure #207-97. The compositional modifications cited in the NADCA procedure are as follows.

_ _ C _ _ Cr ___Mo__ ___P __ ___S ___ 0.37-0.42 5.00-5.50 1.20-1.75 0.025 max 0.005 max

Moreover, NADCA Recommended Procedure #207-97 addresses impact testing of austenitized and double tempered material, wherein short-transverse direction Charpy V-notch (CVN) toughness is determined on samples cut with the base of the notch parallel to the longitudinal direction. Acceptance criteria involve an average CVN value of 8 foot pounds, together with a minimum single test value of 6 foot pounds. It has been suggested that superior toughness is associated with minimization of large, primary carbides. Primary carbide minimization is addressed by utilizing various thermomechanical practices, including high temperature homogenization treatments. Considerable variation in H13 toughness is observed, and conventional H13 products may be produced, in some instances, with levels of toughness consistent with “premium” grades. In this context, Rensselaer is undertaking a rigorous characterization of selected H13 microstructures, with emphasis on carbide size distribution and the relationship of carbides to fracture paths. Four H13 samplings have been made available to Rensselaer by Ellwood Materials Technologies, together with CVN characterization results. This paper outlines the metallographic techniques involved in the initial phase of this project, presents the carbide size characterizations and discusses the relationship of carbide size to the observed CVN toughness. MATERIALS As-tested CVN specimens were received from four heats of H13 material. The heats include three conventional H13 compositions (designated A, B, and C, respectively), and a modified H13 composition with lower silicon, lower vanadium, and higher molybdenum (designated D). The compositions of heats A, B, and C are consistent with premium H13, as called out in NADCA Recommended Procedure #207-97. Heats A, B and D were melted in an Electric Arc Furnace, ladle refined, vacuum

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degassed, and bottom-poured into ingots. Heat C was produced in the same manner, but further refined via Vacuum Arc Remelting (VAR). The chemical compositions are as follows. Heat_A Heat_B Heat_C Heat_D Carbon 0.40 0.38 0.37 0.39 Manganese 0.34 0.34 0.37 0.34 Silicon 0.92 0.87 0.87 0.29 Chromium 5.10 5.18 5.08 5.05

Nickel 0.14 0.07 0.07 0.13 Molybdenum 1.29 1.27 1.28 1.76 Vanadium 0.84 0.83 0.80 0.62 Phosphorous 0.006 0.010 0.009

0.008 Sulfur 0.002 0.001 0.001

0.001 Copper 0.09 0.05 0.04 0.09 Aluminum 0.05 0.03 0.03 0.03

Heats A and B were cast as 34” x 47” ingots, with weights of 28,500 pounds. Heat C was cast as a 33” round VAR ingot. Heat D was cast as a 28” x 36” ingot, with a weight of 15,900 pounds. The cast materials were subjected to varying high temperature homogenization treatments, then forged and annealed. Heats A and C were forged to a 12” x 30” cross-section, heat B to a 10” x 30” cross-section, and heat D to a 12” x 24” cross section. The four forged and annealed products were subjected to CVN characterization, consistent with NADCA Recommended Procedure #207-97. After quenching and double tempering according to the NADCA procedure, the results were as follows. Heat A Heat_B Heat_C Heat_D CVN Results, ft-lbs 5,7,9 18,17,15 16,16,18 19,20,18 Heat A does not meet the NADCA acceptance criterion, whereas heats B, C and D do. METALLOGRAPHIC CHARACTERIZATION

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In order to obtain meaningful information about the microstructure of each of the tool steel samples, a careful metallographic preparation was carried out. The following is a brief description of the techniques used. Representative metallographic samples were prepared from each of the as-received CVN test bars. The section plane corresponded to the plane determined by the longitudinal and width directions of the parent slab, and was roughly parallel to the crack propagation plane in the Charpy test. Before cutting these samples, the fracture surfaces of each of the CVN bars were removed (cut off) in order to prevent contamination and/or physical damage so as to preserve these surfaces for subsequent examination. After removing the fracture surfaces, two small samples were cut from each of the test bars using an aluminum oxide wheel. These samples were then carefully cold mounted in epoxy using a standard procedure. The samples sat overnight in a water bath and were then cured in a furnace at 150 °C for one hour. A swing grinder was used to remove the uneven excess epoxy from the back of the mount as well as provide relatively parallel surfaces for subsequent grinding. Following swing grinding, the samples were placed together in a lapping machine for initial grinding in order to create parallel surfaces. The samples were then manually polished using moderate pressure in the following manner. Each sample was first polished on a series of coarse grit papers of the following grades: 180, 240, 320, 400, 600. Following the initial coarse polishing, fine polishing was carried out on a series of low-nap diamond wheels, for thirty turns each on 15 µm, 9 µm, and 0.1 µm diamond wheels. At this point it becomes important to note that only low-nap and no-nap cloths must be used. Such practice largely avoids the pullout of carbides and inclusions that can occur if a high nap cloth is used. This is due to the brittle nature of these particles, in combination with the potentially poor particle-matrix bonding, associated, in part, with the inherent brittleness of the underlying martensitic matrix (1). The tendency is for the higher nap to essentially grab hold of these particles and pull them out leaving behind holes that manifest themselves as black dots easily visible under low magnification (100X). Each of the samples was color etched using a meta-bisulfate based reagent containing a hundred milliliter solution of one part hydrochloric acid, two parts distilled water, and one gram of potassium meta-bisulfate (2). This reagent was chosen because it colors the underlying tempered matrix gray-blue and leaves the carbides white. Each sample was etched for 35 seconds under continuous motion in order to ensure a uniform etch. Micrographs were obtained with a Polaroid digital camera and with an ausJena Neophot 21 metallograph, augmented with oil immersion technique.

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Figure 1 shows a typical micrograph obtained with the digital camera at a setting of 500x. In addition to such “random” microstructures, areas involving relatively large carbides were observable, as shown in Figure 2. In some cases arrays of carbides were to be noted, as shown in Figure 3. Figure 4 shows the resolution of moderate sized carbides possible with oil emersion technique at 2000x. Due to the contrast it was relatively easy to characterize the carbide network using image analysis software. The image analysis software used was Image-Pro Plus 4.0 with Materials-Pro Analyzer, both from Media Cybernetics. Ten pictures were taken of each of the four samples at random locations with the Polaroid digital camera at a magnification of 500X. Before analysis, each image was modified with flatten and dilation filters, to further increase contrast. The flatten filter was used to even out the background, a technique beneficial in object isolation. The dilation filter was used to enhance the brighter objects, in this case carbides. In order to distinguish between the tempered matrix and carbides, an appropriate threshold intensity was used. The software was then programmed to calculate the average and maximum carbide sizes (among many possible characterization indices), disregarding any carbides smaller than 1 square micron. The results of the software analysis are set forth below. Heat A Heat_B Heat_C Heat_D Mean carbide area, µm2 4.1 1.8 1.5 1.6

Mean carbide length, µm 2.5 1.6 1.6 1.8 Max. carbide area, µm2 92.3 20.7 3.2 4.8

Max. carbide length, µm 15.5 5.0 2.7 3.4 DISCUSSION It is clear that the heat, namely A, with by far the coarsest carbide structure is also the heat that did not meet the CVN test acceptance

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criterion. Even so, the carbide sizes characterized are not in the 20-50 µm range associated with the most inferior product forms. Thus, important effects of more subtle carbide size distributions are clearly suggested. Efforts to minimize the incidence of large carbides have often focused on homogenization heat treatments, although the harmful effects of slow cooling from austenitization have been illustrated in two recent papers (3,4). Moreover, the relatively coarse carbides and low impact properties of Heat A are believed to be related to that material receiving a different homogenization treatment from the others. However, a rigorous discussion of the origin of the carbide size distributions is beyond the scope of this paper, and future work on this project (see below) will focus on the involvement of the characterized carbides in crack propagation. The relatively straightforward carbide characterization methodology set forth in this paper can be used, as needed, to quantify carbide populations in a wide variety of homogenized, annealed and austenitized and tempered microstructures. FUTURE WORK The future work in this Finkl Challenge Program project will include characterization of the size distribution and type of carbides present on the fracture surfaces of the CVN specimens, as well as detailed metallographic study of sections perpendicular to the CVN fracture “plane”. These metallographic studies will evaluate the possible interaction of carbides with the propagation of sub-critical cracks near the Charpy V-Notch. The overall results should allow a sophisticated re-examination of the requirements for achieving elevated toughness in H13 materials. SUMMARY A rigorous metallographic examination of carbide size distribution has been undertaken for three conventionally processed H13 heats, and a modified H13 composition with lower silicon, lower vanadium, and higher molybdenum. One of the conventionally processed H13 heats displayed Charpy V-Notch values below the acceptance criterion of NADCA Recommended Procedure #207-97. This same material contained markedly larger carbide size distributions than displayed by the three other, acceptably tough materials.

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REFERENCES 1. Bjerregaard, L., K. Geels, B. Ottesen, and M. Ruckert, Metalog Guide, Struers, 1992, p. 74. 2. Beraha, E. and B. Shpigler, Color Metallography, American Society for Metals, 1977, p. 31. 3. Brada, G., Die Casting Engineer, May, 2002, p. 22. 4. Sandberg, O., B. Klarenfjord and P. Miller, Die Casting Engineer, May, 2002, p. 40.

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Figure 1. Typical digital light micrograph, from Heat C, showing white-appearing carbides, of relatively modest size.

Figure 2. Digital light micrograph showing large carbide, from Heat A.

20µ

20µ

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Figure 3. Digital light micrograph showing large carbide and carbide array,

from Heat A.

Figure 4. Light micrograph taken with oil immersion technique, showing carbides from Heat A.

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20µ