RESEARCH PROJECT No. 32 IMPROVING THE PROPERTIES OF AUSTEMPERED DUCTILE IRON DIS Research Project No. 32 by R. B. Gundlach Climax Research Services / DUCTILE IRON \ SOCIETY Issued by the Ductile Iron Society for the use of its Member Companies - Not for General Distribution DUCTILE IRON SOCIETY 28938 Lorain Road North Olmsted, Ohio 44070 (440)734-8040 April 2001 RESEARCH PROJECT NUMBER 32 IMPROVING THE PROPERTIES OF AUSTEMPERED DUCTILE IRON DIS Research Project No. 32 R. B. Gundlach Climax Research Services / DUCTILE IRON \ SOCIETY Issued by the Ductile Iron Society for the use of its Member Companies - Not for General Distribution DUCTILE IRON SOCIETY 28938 Lorain Road North Olmsted, Ohio 44070 (440)734-8040 April 2001 IMPROVING THE PROPERTIES OF AUSTEMPERED DUCTILE IRON DIS Research Project No.32 R. B. Gundlach Climax Research Services March 19,2001 SUMMARY An investigation of the influence of austenite content on the properties of austempered ductile iron was undertaken. The "reacted" austenite content in the austempered structure was controlled by varying austenitizing temperature and, thus, the carbon content of the austenite during austenitizing. Ductile iron plates measuring 1.25 by 8 by 10 inches and containing 3.4C- 2.7%-0.26Mn-l.lNi-0.16Mowere austempered at 725F (385C). The properties evaluated included tensile properties, machinability and thermal expansion coefficient. Studies in a laboratory dilatometer were conducted to determine the transformation behavior of the ductile iron alloy with the intent to vary the carbon content during austenitizing. The lnvestigation included "step-austenitizing" in which the material was first heated above the upper critical temperature and subsequently cooled and held below the critical temperature. The experiments demonstrated that the carbon content of the austenite could be controlled by varying austenitizing temperature, even at temperatures below the upper critical temperature (a-transus). The stability of austenite, when cooled down into the intercritical region was found to be high, with no proeutectoid ferrite formation occurring; and specimens were successfully austempered from temperatures (1440Fl782C) well below the a-transus without pearlite or ferrite formation preceding the bainite reaction. Intercritical heat treatments were also performed with the intent to produce a mixed ferrite + ausferrite microstructure. Heat treatment in the intercritical (three-phase) region showed that an austempered structure of 50% proeutectoid ferrite + 50% ausferrite could be achieved when austempering from 1450F (788C). Austempering of the plate castings was performed at 725F (385C) from austenitizing temperatures above the critical temperature and by step-austenitizing to temperatures below the upper critical temperature. Austenite content in the austempered structure was found to decrease with decreasing austenitizing temperature. Tensile properties were found to generally increase with decreasing austenitizing temperature. Machinability in drilling was found to improve significantly as austenite content was reduced. The thermal expansion coefficient also decreased with decreasing austenite content. Austempering of the plate castings was also performed at 725F (385C) from an intercritical temperature [1450F (788C)l below the upper critical temperature. Tensile strength was reduced but tensile elongation was quite high. Machinability in drilling was found to improve significantly, comparing quite favorably with pearlitic (grade D5506) ductile iron. DIS Research Pro~ect#32 The benefits of lower final or reacted austenite content in AD1 structures were clearly demonstrated in this investigation. The results of austenitizing at temperatures near, and below, the a-transus produced lower reacted austenite contents and showed significant improvements in properties. The problem with formation of free ferrite when austempering from lower austenitizing temperatures should be addressed to better define the potential for step- austenitizing prior to austempering. The properties obtained by austempering directly from the intercritical temperature range were also quite interesting. While the properties of this material did not meet the specifications of any of the AD1 grades, the good strength, high ductility and excellent machinability observed in this material are very encouraging and suggest that further investigations are warranted. The particular combination of properties observed for this material suggests that the material should be considered for high toughness applications. The material should be evaluated more thoroughly with a range of heat treatments (and ferritelausferrite ratios) to further develop the structure-property relationships. Fracture toughness and fatigue properties should also be evaluated to better define capabilities. Paee 2 of 40 DIS Research Project #32 INTRODUCTION Historical investigations in the development of austempered ductile iron (ADI) showed that, for a given austempering temperature, properties vary greatly with variation in composition. Likewise, several investigators have demonstrated that properties vary with austenitizing temperature. Variations in composition and austenitizing temperature clearly influence the austenite content of the austempered structure through their effects on carbon solubility and austenite stability. While specifications for AD1 grades, including ASTM 897 (97), specify minimum properties such as tensile strength, yield strength, elongation and hardness, AD1 properties can well exceed these minimums. Many have learned how to improve toughness and ductility through improving the quality of the base material, i.e., achieving higher nodule count and nodularity and reducing levels of microshrinkage, microsegregation, carbides and inclusions. Some corporate specifications go so far as to specify levels of some of these microstructural features for castings that are to be austempered. No one, to this investigator's knowledge, has attempted to specify (control) the final austenite content, that is, the "reacted" austenite content. Improving Strength in AD1 It seems clear that much of the variation in yield strength and tensile strength among various alloyed AD1 materials investigated is related to the volume fraction of austenite in the AD1 structure. The data in Table 1 illustrate the range of propertiqs observed in twelve ductile iron alloys austempered under the same heat treatment conditions . Note the wide range in strength; particularly yield strength, among the twelve alloys. Table 1 Tensile Properties of Twelve AD1 Alloys with Identical Heat Treatments (Reference 1) [Austenitized 1600F (872C) + Austempered 700F (371C)] Tensile Yield Strength Strength Percent. Alloy ksi ksi Elongation Range 141-171 110-138 3.4-14.5 Mean 155 121 7.8 Std Dev 8.3 9.2 2.8 Paee 3 of 40 DIS Research Project #32 Fig. I Influence of austenitizing temperature on the carbon content of the austenite matrix for 2.5%Si ductile iron alloys, after verhoeven3. It is this investigator's contention that the variations in yield st;ength are due, in large part, to significant variations in austenite content. Work by Hayrynen showed that yield strength was governed by ferrite grain size, however, it is suspected that yielding is governed both by grain size and the amount of austenite. It would appear that less austenite leads to a higher flow stress. Preliminary examinations of data from the literature indicate that certain elements increase yield strength for a given heat treatment, while others decrease yield strength. Ductile iron alloys that contain Ni, which stabilizes austenite, have often displayed lower yield strengths than unalloyed ductile iron. One company recently inquired about obtaining AD1 with a combination of high yield strength (140 ksi) and high elongation (7%). A review of the literature indicated that five different investigators reported achieving this exceptional combination of properties in ADI. It occurred to us that yield strength could be raised significantly for a given austempering temperature, if compositio~andlor austenitizing conditions were adjusted to reduce the austenite content. As Verhoeven demonstrated in the attached Figure 1, modification of austenitizing conditions was effective in reducing dissolved carbon content. Preliminary experiments with variations in austenitizing temperature, using castings of AD1 grade 1.5, proved that yield strength could be raised by 15,000 psi with no apparent loss in ductility. Tensile 0.2% Yield Strength Strength Elongation Red. in Condition ksi ksi % Area, % Std. Heat Trt 166.0 124.0 10.6 10.1 Modified Ht Trt 175.6 138.6 11.3 10.1 Page 4 of 40 DIS Research Project #32 The laboratory ex eriments listed above were quite encouraging. At the same austempering temperature, yiel8 strength continually pcreased with decreasing austenitizing temperature, yet there-was no significant change in ductility. The potential for further imp~ovementthrough additional modification of heat treatment a pears psssible. Even greater improvement might be achieved through optimization of chemica f'composition. Improving Machinability in AD1 The use of AD1 has been somewhat hampered by the difficulties associated with machining it. It has been well established by various investigators that the austenite in ausferrite structures both work-hardens and transforms to martensite during machining. Methods to overcome the machinability problems most often involve larger depths of cut in order to undercut the work- hardened