The Effect of Cryogenic Processing on the Mechanical ...

Proceedings of the 26th ASM Heat Treating Society Conference B.L. Ferguson, R. Jones, D.S. MacKenzie, and D. Weires, editors

The Effect Of Cryogenic Processing On The Mechanical Properties Of Austempered Ductile Cast Iron (ADI) Susil K. Putatunda, Codrick Martis Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202, USA [email protected] Frederick Diekman Controlled Thermal Processing Inc. Park City, Illinois 60085, USA [email protected] Rozalia Papp Air Liquide US LP, Countryside, Illinois 60525, USA [email protected]

Abstract An investigation was carried out to examine the influence of cryogenic processing on the microstructure and mechanical properties of Austempered Ductile Cast Iron (ADI). ADI has emerged as a major engineering material in recent years because of its many attractive properties. These include high yield strength with good ductility, good fatigue strength, fracture toughness and wear resistance. In this investigation, compact tension and cylindrical tensile specimens were prepared from ductile cast iron as per ASTM standards and were austempered at a lower bainitic temperature of 288°C (550°F). These specimens were then cryogenically processed. The mechanical properties and fracture toughness of these samples were evaluated and compared with the noncryogenically treated samples. The influence of cryogenic heat treatment on the microstructure of these samples was also examined. Test results show that the cryogenic processing can improve the mechanical properties without compromising the fracture resistance of the material.

shorter heat treatment processing cycles. Moreover, the density of ADI is about 10% lower than steel. Thus it has the added advantage of higher specific strength compared to wrought or forged steel. Because of these advantages, the use and application of ADI continues to grow worldwide and its market share is going to exceed more than several billion dollars in coming years. In recent years there has been significant interest in cryogenic processing of materials. There have been several applications of cryogenic processing of materials for aerospace and other industries. This process has been used for aircraft components like turbine blades, landing gear systems, gears, cutting tools etc. Cryogenic processing has also been used to treat a variety of other materials such as nickel based super alloys, and even nonmetallic materials. Cryogenic processing has been found to improve the mechanical properties of these materials. The conventional cryogenic processing involves subjecting materials to extremely cold temperature (in the range of -80 to -155°C) and then heating them to above room temperature. The cryogenic heat treatment cycle consists of several steps such as (a) Ramp down, (b) Hold, (c) Ramp up and (d) Tempering. During ramp down, the temperature of the material is cooled down to a very low temperature from ambient temperature very slowly. This slow cooling helps to reduce the temperature gradient within the component and keep stresses to a minimum. During holding period the temperature is held at the cryogenic temperature for a predetermined time period and in the ramp up stage the temperature of the bath is brought back to ambient temperature slowly. Finally the material is tempered at slightly elevated temperatures.

Introduction The focus of this investigation was to examine the influence of cryogenic processing on the microstructure and mechanical properties of Austempered Ductile Cast iron (ADI). ADI has emerged as a major engineering material in recent years because of its excellent properties such as high strength, with good ductility [1-4], good fatigue strength [5-10], fracture toughness [11-17] and excellent wear resistance [18-20]. It is now used extensively in many structural applications such as automotive components, locomotive wheels, gears, crankshaft, connecting rods, brake shoes etc. ADI has other advantages such as low production cost because of its good castability and excellent machinability and consequently longer tool life and

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While cryogenic processing has been successfully used to improve the mechanical properties of various materials [2122], its use in cast iron and ADI has been rather limited. This investigation was undertaken to explore the use of cryogenic processing in ADI and the objective of this investigation was to examine the influence of cryogenic processing on the microstructures and mechanical properties of ADI.

temperature in 8 hours. The samples were then tempered at +150°C.

Tensile Testing Tensile testing of these samples was carried out as per ASTM standard E-8 [23]. The tests were carried out at a constant engineering strain rate of 4x10-4 s-1 on a servo hydraulic MTS (Material Test System) test machine at room temperature and ambient atmosphere. Load and displacement plots were obtained on a X-Y recorder and from these load-displacement diagrams, yield strength, ultimate tensile strength and % elongation values were calculated. Four samples were tested from each heat treated condition and the average values are reported in this paper.

Experimental procedure Material The material used in this investigation was an unalloyed nodular cast iron. The chemical composition of the material is reported in Table 1. The material was cast in the form of KEEL blocks and from these cast blocks, cylindrical tensile specimens and compact tension specimens for fracture toughness tests were prepared as per ASTM standards E-8 [23] and E-399 [24] respectively.

Fracture Toughness Testing After heat treatment, the compact samples were ground and then polished with 600 grit emery paper. The specimens were then pre-cracked in fatigue at ΔK level of 10 MPa√m with a load ratio R = 0.10 to produce a 2 mm sharp crack-front in accordance with the ASTM standard E-399 [24]. After fatigue precracking, the specimens were loaded in tension in a servohydraulic MTS test machine and the load-displacement diagrams were obtained with a clip-gauge in the knife edge attachment on the specimens. From these load displacement diagrams, PQ values were determined using the 5% secant deviation technique as per ASTM standard E-399 [24]. From these PQ values, KQ values were determined using the standard stress intensity factor calibration function for the compact tension specimens. Since these KQ values satisfied all the requirements for a valid KIC test as per ASTM standard E-399, they are all valid KIC values. Four samples were tested from each heat treated conditions and the average values are reported here.

Table 1: Chemical composition of the material Element

wt %

C

3.6

Si

2.5

Mn

0.4

S

0.015

P

0.015

Mg

0.010

Fe

93.46

Metallography and X-ray Diffraction Studies Microstructures of all the samples were examined by optical microscopy after polishing and etching with 5% nital solution. X-ray diffraction (XRD) analysis was performed to estimate the austenite content and the carbon content of austenite following the procedure of Rundman and Klug [25]. XRD was done using a monochromatic copper Kα radiation at 40 kV and 100 mA. A Rigaku rotating head anode diffractometer was used to scan angular 2θ range from 42°-46° at a scanning speed of 0.25° per minute and in 2θ range of 72°-92° at a scanning speed of 1° per minute. The profiles were analyzed using Jade 5 software to obtain the peak positions and the integrated intensities of {111}, {220} and {311} planes of FCC austenite and {110} and {211} planes of BCC ferrite. The volume fractions of ferrite (Xα) and austenite (Xγ) were determined by the direct comparison method using the integrated intensities

Heat Treatment After fabrication the samples were austempered as follows. The sample were initially austenitized at 927°C (1700°F) for 2 hrs and then subsequently quenched in a molten salt bath maintained at 288°C (550°F) .The samples were then austempered at this temperature for 2 hours. After austempering the samples were treated cryogenically as follows; The samples were initially cooled down to -150°C from ambient temperature very slowly i.e. 8 hour ramp down to 150o C and held at this temperature for twelve hours. After that the temperature of the samples were increased to ambient

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of the above planes [26]. The carbon content of the austenite was determined by the equation [27] a = 0.3548 + 0.00441 C … (2) Where a is the lattice parameter of austenite in nanometer and Cγ is the carbon content of austenite in wt%. The {111}, {220} and {311} planes of austenite were used to estimate the lattice parameter. Three samples were examined from each heat treated condition and the data reported is the average from these samples.

Results and Discussion Influence of Austempering Temperature on the Microstructure The microstructure of the as-cast sample is reported in the Figure 1. It shows a predominantly pearlitic structure with graphite nodules dispersed in it. The graphite nodules were well rounded with nodularity of about 85%. The microstructure of the austempered samples is reported in Figure 2 while the microstructure of the samples processed by cryogenic heat treatment is reported in Figure 3. The microstructures of Austempered samples show a mixture of bainitic ferrite and austenite. It had the appearance of a lower bainitic microstructure. The ferrite appears as dark needles where as austenite appears as white in the micrograph. The austenite has the appearance of slivers between finite needles. The microstructures of cryogenically heat treated samples consisted of ferrite and tempered martensite. The austenite was almost non- existent in the cryogenically treated samples. Moreover some precipitation of carbide was observed in these samples. During austempering process, ferrite first nucleates out of austenite by the nucleation process and then grows with austempering time. As the ferrite grows, the remaining austenite becomes more and more enriched with carbon.

Fig 2: Microstructures of samples austempered at 550°F

Fig 3: Microstructure of austmpered and cryogenically treated samples. Table 2 reports the microstructural constituents of the material in both austempered and cryogenically treated samples as determined by X-ray diffraction technique. While austemepred samples had 8.33% austenite, in the cryogenically treated samples no austenitic peak was observed. This indicates that the austenite had transformed into tempered martensite in these samples. The microstructure in both the austempered and cryogenically treated samples were very fine scale in nature. The ferritic cell size (d) was determined for austempered samples using the well-known Scherrer equation [26]. This is also a measure of the mean free path for the dislocation motion in the material. Table 2 reports the ferritic cell size of the austempered sample. The ferritic cell size in the austempered samples were about 31.54 nm. This fine scale ferrite and austenite has caused significant strength in the austempered samples [28-29]

Fig 1: Microstructure of as-cast ductile cast iron samples.

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Table 3: Mechanical Properties of the Material

Table 2: Microstructural Constituents of the Material

Material condition

% Retained Austenite Content,

Carbon Content of Austenite, Cγ (%)

XγCγ

8.33

1.5619

0.13008

Material condition

Hardness (RC)

Yield Strength (MPa)

Tensile Strength (MPa)

% elongation

Austempered samples

37.5

1198.5

1328

1.92

Cryogenically treated samples

41.4

1303

1466

1.4

d (nm)

Xγ

Austempered

Ferritic cell size,

31.54

Influence of Austempering Temperature on Mechanical Properties

treated samples did not have any austenite left its strain hardening exponent was lower. Since the wear resistance is directly related to the hardness of the material the cryogenically heat treated ADI will have higher wear resistance also.

Table 3 compares the mechanical properties of Austempered and cryogenically heat treated samples. It is evident that the hardness of the cryogenically treated samples are higher than the austempered samples. The table also shows that cryogenically treated samples have higher yield and tensile strength than the austempered samples. On the other hand, the ductility of the material has decreased as a result of cryogenic heat treatment. This increase in hardness and strength in the cryogenically treated material is due to the absence of austenite content and presence of carbides in these samples. The cryogenic heat treatment has converted the retained austenite of the austempered samples in to a tempered martensitic structure with some precipitation of carbides. Figure 4 shows a micrograph of the sample indicating the presence of carbides. Austenite is a FCC (face centered cubic) and soft and ductile phase. Due to the presence of about 8.33% austenite in austmepered samples, the ductility of these samples was higher. On the other hand in cryogenically treated samples virtually no austenite was left and some carbide precipitation has taken place. Consequently the strength and hardness of these samples were higher and ductility was lower.

While it is now a well-known fact that the cryogenic treatment can improve the mechanical properties of the materials there is still a controversy about its origin. Several investigators [3032] have suggested that improvement in the mechanical properties occur due to austenite to martensitic phase transformation. However, this improvement in properties has been observed in non-ferrous material also [33], where austenite was absent and hence no austenite to martensitic phase transformation can take place. Other investigators have suggested various other contributing factors for improvement in mechanical properties such as precipitation of very fine scale carbide [34], reduction of crystal defects such as dislocations and vacancies [35] and presence of compressive residual stress [36].

The strain hardening exponent of the austempered and the cryogenically treated samples are reported in Table 4. This table shows that the cryogenically treated samples had a lower strain hardening exponent compared to the austempered samples. This is because the austenitic carbon content (XγCγ) of the Austempered samples were significantly higher. The austenitic carbon content is a very important microstructural parameter. This is a measure of the total carbon content of the austenite. As this parameter increases the strain hardening exponent of the material will increase [20]. Higher austenitic carbon promotes greater interaction between dislocations and solute carbon atoms in ADI. This leads to higher strain hardening exponent of the material [20]. Since cryogenically

Fig.4: Microstructure of the cryogenically treated sample showing the presence of carbide.

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While all these factors need to be investigated thoroughly, the test data of this investigation indicate that at least in ADI transformation of austenite to martensite and precipitation of carbides take place and contributes to improvement of mechanical properties of ADI. The fracture toughness values of the austempered and cryogenically treated samples are reported in Table 4. It is evident that the fracture toughness values of both these batches of samples are very similar. It is interesting to note that even though the cryogenically treated samples had higher yield and tensile strength (about 8% higher) the fracture toughness of the cryogenically treated samples was only 2% lower. Generally in all structural materials as the yield or the tensile strength increases the fracture toughness decreases. However, it appears that cryogenic treatment can improve mechanical properties of ADI without compromising the fracture toughness of the material.

Fig.6: Fractograph of the non-cryogenically treated samples.

Conclusions Unalloyed ductile cast iron was austempered in the lower bainitic temperature range of 288ºC (550ºF) and then cryogenically processed. Cryogenic processing has resulted in improvement in mechanical properties such as hardness, yield strength, ultimate tensile strength of the material compared to austempered samples. This improvement in mechanical properties can be attributed to precipitation of some carbides and transformation of austenite to tempered martensitic structure. Moreover, the improvement in mechanical properties occurred without any significant reduction of the fracture toughness of the material. Thus it appears that a unique opportunity exists for material processing for ADI by cryogenic treatment i.e. it can be used to improve the mechanical properties without compromising the fracture resistance of the material.

Table 4: Fracture Toughness of the Material. Material condition

Fracture Toughness, (MPa√m)

Strain Hardening exponent (n)

Austempered

59.65

0.063

Cryogenically treated

58.4

0.053

Fractograph

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The fractographs of the cryogenically treated samples are shown in Figures 5 and 6 respectively. Both these samples show the presence of quasi cleavage type of fracture with occasional dimples. These dimples are mostly at the graphite nodules/matrix interface. Basically no significant difference was observed in this fracture mode of the samples.

Fig.5: Fractograph of the cryogenically treated samples.

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