Effect of Different Quenching Media on Microstructure, Hardness, and Wear Behavior of Steel Used in Petroleum Industries

Medium carbon steel always used in most parts of the oil and gas industry, due to their cheap, simple manufacturing and easy deformation. However, as-rolled medium carbon steel usually does not meet the demands for many petroleum applications where high hardness and strength are needed; mainly because of their limitations in some mechanical features. Hence, many methods of thermal treatment techniques were adopted with a view to manipulate its structure and thus extend its scope of applications.This study focus on using a proper heat treatment to enhance medium carbon steel properties. Quenching and tempering treatments are usually the end of any manufacturing process to production of a machine component. However, the internal and residual stresses which generate during the heat treatments due to the non-uniform the quenched layer depth lead to distortion or cracked the machine component result in deteriorate the properties. Thus, the selection of optimum parameters of the quenching process is very important to ensure the achievement of the desired properties of the machine component. The main aim of this study is to get the optimal conditions for heat treatment which brought the best combination of mechanical properties of medium carbon steel without tempering treatment. Thus, in current study, an investigation of effects of polymer solution media on the quenching process was carried out with samples made of medium carbon steel. The samples were quenched in four different concentrations (0, 10, 15, 20) % of polymer (polyalkylene glycol) solution and set of quenched samples were tempered at 400 C for 1h. The resulting as-quenched and tempered microstructures were compared and evaluated. Mechanical properties (hardness and wear rate) of the samples were determined using suitable standards. The results show that the quenched samples in polymer solution with 10% polyalkylene glycol were brought the best combination of mechanical properties as compared with other quenched and tempered samples.


Introduction
Medium carbon steels have increased usage in petroleum industry. Their applications include flow lines, structural components, platforms, jackets, and pipelines. [1]. Medium carbon steels have excellent response for heat treatment. Therefore, thermal treatment is the effective tool which most widely used in improving the properties of medium carbon steel [2]. Among all thermal treatment techniques hardening treatment has the largest effect in enhancing the mechanical properties of medium carbon steels by quenching in appropriate medium.
The purpose of the most quenching treatments is to provide a fast cooling rate which is higher than the critical cooling rate in a specified depth of the component to obtain materials which exhibit increased hardness without distortion or cracking defects [3].

E 200
There are many quenchants which are used in industry. Water and oil were conventionally the most commonly used quenching media in the thermal treatment processes to harden steel alloy, especially for mild and medium carbon steels because they are readily quenchable [4,5]. However, because of the water quenched steel needs to further treatment (tempering) to obtain the desired properties which means additional operation and production cost, and with arising the environmental, disposal, safety and toxicological concerns, there are grown interests in the potential use of alternative quenching technologies [6,7]. One of the most commonly considered alternatives to quench water or oils are polymer solutions [8,9]. Beside the supplying substantially higher safety with respect to fire and disposal, polymer solutions can supply more regular heat removal through quenching treatment leading to decreased thermal gradient and decreased distortion. In addition to, quenching in polymer solutions have another advantages in reducing the distortion and internal stress in quenched parts, minimize the cost, and quenched parts can be easy to clean. [10] In this work, the polyalkylene glycol was used as polymer quenchant. The main aim of this study is to study the possibility of get a quenching media produces mechanical properties which is equivalent to those resulting from quenching in water with tempering. For this purpose, the effects of varying concentrations (0, 10, 15, 20) % of polymer in water on mechanical properties (hardness and wear rate) of medium carbon steel were investigated. Also, the optimal heat treatment conditions which brought the best combination of mechanical properties of medium carbon steel was determined.

Sample material
Samples from medium carbon steel were used in this work. The chemical composition of used material is list in Table (1) polyalkylene glycol with density of 0.98 g/ml and viscosity of 32 cSt was used as quenchant to achieve the current work  Fig. (1) summarized the whole experimental procedure.

1-Microstructure test
The as-received and quenched samples will be prepared by mounting, grinding, polishing and then etched to analyze the microstructure, and observe the change in grain structure. The optical microscope was used to perform this test.

2-Hardness Measurement
The hardness of the specimens was measured with a Vickers hardness test instrument. The hardness tests were performed under an indentation load of 300 N for 15 s. In order to obtain a reliable statistical data, analysis points were spaced so as to eliminate the effect of neighboring indentations, and the hardness was evaluated by taking three indentations on each specimen and averaging hardness was calculated. Fig. (2) illustrate the hardness sample.

3-Wear rate test
Abrasive wear tests were performed on samples in according to the ASTM G99 using pin on disc tester device with a continuously rotating alloy steel disc with hardness 55 HRC as a counter surface.
The wear sample was cylindrical with 10 mm diameter and 20 mm height (Fig. 2). The wear test was done under constant load of 10 N, speed of 2 cm/s, and sliding distances of 50 m. After the end of each cycle of wear test, the mass of the worn out samples was determined by using a digital balance to obtain the weight lost. Weight lost from the tests was used to determine the wear rate W.R by using the following equation [11]: Where: is wear rate (cm 3 /cm), is weight loss in (mg), is sliding distance in (cm), is sample density (7.86 mg/cm 3 ), is sample weight before and after wear test (mg).

2-Hardness
The hardness of any materials is mainly associated with the properties of its ingredients, and it can be controlled by tailoring the chemical composition, microstructure, forming, and thermal treatments.

3-Wear rate
The wear rate values of the as received, quenched, and tempered samples are given in Figure 5. In general, wear rates for both quenched and tempered samples were reduced, this behavior coincides with the increasing in surface hardness which is attributed to the formation of hard martensite phase, this mind is correlated with AbdulKhaliq F. Hamood et. Al [12]. Also, Fly et. Al [13] reported that the wear resistance depends only on the skin properties. Wear rate of tempered samples marginally increases with respect to that of quenched samples due to decreasing of martensite phase and Overall, the wear rates were stable in the polymer solution quenched samples at all sliding distances. This is assumed to be due to the hard martensite phases were embedded in retained austenite structure, which reduce the risk of martensite cracking, that in turn contributed to the reduce of wear rate.

Conclusion
It can be concluded from the results that the best combination of hardness and wear resistance properties of medium carbon steel can be achieved by hardening in polymer solution with 10% polyalkylene. The overall mechanical properties of samples quenching in this solution were better than that quenching in water and tempered. Therefore, it can be improved the properties of medium carbon steel and reduces the time and cost of hardening treatment by using hardening in polymer solution with 10% polyalkylene instead of hardening in water and tempering.