Effect of Cryogenic Treatment on Mechanical Properties of AISI 4340 and AISI 4140 Steel

From last epoch till to date, AISI 4340 and AISI 4140 have been widely used in different engineering applications. These applications include bolt, screws, gears, drive shafts, crane shaft and piston rods for engines due to its upright mechanical properties, cost-effective and easily available in market. In present work, deep cryogenic treatment effect on the mechanical properties of AISI 4340 and AISI 4140 have been studied. Present work was carried out at laboratory scale and can be extended for mass production. Our work is simple, straight forward safe and economical. In our work, samples were heat treated in simple muffle furnace and followed by cryogenic treatment in liquid nitrogen. Before cryogenic treatment, all samples were normalized at 860°C to obtain homogenized microstructure. Samples were also compared conventionally heat treatment with quenched in oil quenchant. Experimental results showed that after cryogenic treatment with tempering treatment, one could easily increase the tensile strength, impact toughness and hardness. Advanced optical microscopy (IMM 901) and SEM (Scanning Electron Microscopy), FIT Quanta 200 methods have also been deployed to reveal and interpret the internal structure of samples. It was found from microstructure that cryogenic treated sample increases the impact strength, hardness and tensile strength as compared conventional heat treated quenching approaches.


INTRODUCTION
F rom 1930-1940s, to improve the performance of cutting tool steels, cryoprocessing was used as the method of conventional heat transfer [1].
Cryogenic treatment is now used to increase the mechanical properties like wear resistance, hardness, tensile strength, impact toughness, reduction in residual stresses of materials. The low temperature heat treatment methods are two types: in "cold treatment" the temperature used about -80°C, where as in other method "cryogenic treatment" the temperature used is about -196°C and quenching media used as liquid nitrogen.
Baldissera [2] has reported that after deep cryogenic treatment on tool and die steel grades, the toughness values are improved for these steel grades. The

Effect of Cryogenic Treatment on Mechanical Properties of AISI 4340 and AISI 4140 Steel
precipitation of fine (eta) carbides has been reported after deep cryogenic treatment in AISI M2 [3], T1 [4] and AISI H13 [5]. In a previous work on AISI 4340, a decrease in toughness after deep cryogenic treatment has been investigated due to an increase in the martensite phase fraction [6].According to researchers a decrease in the retained austenite volume fraction with increasing soaking time in deep cryogenic treatment has also been observed [7][8].
The researchers accepted that the decrease in volume fraction of retained austenite gives improved effect on mechanical properties after deep cryogenic treatment of steels [9]. The main reason for the lower mechanical properties of high-speed steel subjected to the cold treatment is the smaller quantity of secondary martensite when retained austenite is transformed in the process of standard tempering [10].
Zablotskii et. at. [11] studied the macrostructure of several steel and found that most of the retained austenite in steels was transformed to martensite during cooling to -80°C and a small portion during cooling to -196°C. It was believed that transient carbide called -carbide precipitated from the decomposition of martensite during tempering after deep cryogenic treatment. These fine carbide particles enhance the strength and toughness of the martensite matrix and also improve wear resistance . Another claim is based on the strengthening of the material brought about by precipitation of fine carbides because of the cryogenic treatment [14].
In this experimental work, the mechanical properties i.e. hardness, tensile strength and impact toughness of medium carbon alloy steel after deep cryogenic treatment are studied [15]. The steel grades AISI 4340 and AISI 4140 are used due to their structural applications. At low temperature processing an increase in carbide formation increases the hardness of medium carbon low alloy steel.
Carbides are normally very hard and brittle due to their brittleness it should be dispersed in the soft matrix. The fine precipitation of carbides in martensitic matrix leads to an increase in hardness, impact toughness and tensile strength of medium carbon low alloy steel (i.e. AISI 4340 and AISI 4140) [16]. The cryogenic treatment in liquid nitrogen is also responsible for phase transformation from retained austenite to primary martensite which not possible in conventional quenching (i.e. austenite and oil quenched) [17].

EXPERIMENTAL WORK
The AISI 4340 and AISI 4140 were normalized in the heat treatment furnace (PLF 120/10

Hardness
The hardness testing was done with the help of Rockwell hardness testing machine using scale C (HRC  shown in Fig. 3(a) and Table 5. As these hardness values were obtained before tempering, the slight improvement in hardness value is due to the transformation of a large fraction of retained austenite to the BCT (Body Centered Tetragonal) structure of martensite.After tempering, there is small decrease in hardness as shown in Tables 6-7 and Fig. 3(b) respectively. The decrease in hardness after tempering is due to the formation of tempered maternsite and carbides.  Tables 8-9 and Fig. 4(b). The impact strength for AISI 4140 after cryogenic treatment and then tempering at 200 and 300°C become increases as 17.5 and 27 J respectively, which is greater than conventional quench and tempered sample for the same tempering temperatures as shown in Table 10.

Tensile Testing
Both conventional and cryogenic treated samples were tested for tensile testing according ASTM standard  Fig. 4(a) shows the impact toughness of charpy impact test of AISI 4340 and AISI 4140 steel. Similar to the effect of cryogenic treatment on hardness, an attempt was made to isolate the effect of tempering from the cryogenic process. This increase in impact toughness after cryogenic treatment can be attributed to an increase in the amount of tempered martensite that transformed from the retained austenite. In fact, the harder and so finer ductile matrix, leads to the higher impact toughness values obtained. As was already that increase in the amount of tempered martensite is responsible for the higher hardness and impact toughness.

Effect of Cryogenic Treatment on Mechanical Properties of AISI 4340 and AISI 4140 Steel
ASTM-E8. Fig. 5(a-b)and Table 11 show the experimental results of tensile strength for oil quenched and cryogenic treated samples respectively. From Fig. 5(a-b) and Table   11, it is clear that there is an increase in tensile strength after cryogenic treatment of AISI 4340 and AISI 4140. The increase in amount of martensite after cryogenic treatment gives rise to the tensile strength of medium carbon steel alloys as shown in Table 11.

Optical Microscopy
The  Fig. 6(b). The white region in Fig. 6(a-b)    Similar mechanism has been observed for AISI 4140 and shown in Fig. 6(g-l).

Scanning Electron Microscopy
The surface of AISI 4340 and AISI 4140 steel samples were analyzed for both conventional and cryogenic treatments using scanning electron microscopy. The

Fractography
There are many types of fracture happen by a process involving crack initiation, crack propagation and breaking (fracture). The fracture surface of AISI 4340 steel was examined after tensile testing using SEM to determine the fracture mode as shown in Fig. 8. It has been observed from Fig. 8(a-b), the fracture surface of conventionally treated AISI 4340 steel, crack was initiated at the center of the specimen possibly due to embrittlement of martensitic structure start to propagate and finally fracture occur.
The fracture surfaces of the cryogenic treatment specimens have been compared with those conventionally treated. Fig. 8  initiation process for the specimens tested in the residue paste is linked with the development of corrosion pits on the sample surface. Fig. 8(e) shows an area near the fracture surface of a sample tested. Cracks can be seen emitting from the pits. Coalescence of these all cracks normal to the tensile axis has been observed. A magnified view of the center of the specimen as shown in Fig. 8(h) dimple rupture and tearing shows ductile failure mode in medium carbon low alloy steel.