Performance Evaluation of Deep Cryogenic Treatment on M2 HSS Tool Steel

*Aaditya Patel, Yagnesh Mehta, Jay Chauhan, Yogesh Dabhi, Unnati Joshi and Sanjiv Rajput* 

#### **Abstract**

In the present work, we have attempted to develop a setup that aims at performing cryogenic treatment in a way that gradually reduces the temperature and avoids the thermal cracking of tools due to the large temperature gradient. For experimental purposes, deep cryogenic treatment (at 77 K) was performed on M2 High speed steel and machining studies were conducted on EN-9 material by means of both deep cryogenically treated and untreated tools. Micro-structural examination, hardness and tool wear were studied with the help of a metallurgical microscope, Rockwell hardness and toolmakers microscope respectively. The results reveal that fine carbide precipitation occurring at cryogenic temperature is responsible for enhancing the wear resistance property of the tools. Results also show that there was no noteable change in the hardness values of the tools.

**Keywords:** cryogenic treatment, liquid nitrogen, M2 HSS, carbide precipitation, tool wear

#### **1. Introduction**

In a world of advanced technologies, the requirement for materials to possess properties such as high wear resistance, superior strength, light weight and the ability to perform under extreme conditions is increasing. Thus, there is an indirect demand to develop tool materials with greater resistance and strength that are able to cut work piece materials with higher resistance. The necessity to increase the efficiency and effectiveness of production of a product resulted in emergence of new tool materials such as cermet, ceramics, cemented carbide and ultra-hard materials. In spite of the fact that High Speed Steel (HSS) was developed more than a 100 years ago, it is still widely used in modern day industries. Its major applications include taps, drill bits, broaches, single point cutting tools and also in place of carbide tools with fairly low economical cutting speeds [1, 2].

The property of the material mainly depends on the chemical composition and microstructure. So to alter the properties of a material either the chemical composition or microstructure must change. Heat treatment is the process that is utilised to change the microstructure of the material [1–3]. The regular heat treatment process for tool steels includes heating the raw material up to the austenitizing temperature and then carrying out suitable quenching methodology to achieve the required

*Performance Evaluation of Deep Cryogenic Treatment on M2 HSS Tool Steel DOI: http://dx.doi.org/10.5772/intechopen.81083* 

hardness. In the quenching process, the Austenite (soft phase) present in the steel is converted to Martensite, which is considered the hardest phase when compared to other phases. One of the key aspects of this process is the formation of Martensite, which starts and finishes at a particular temperature. In regular heat treatment, the lowest temperature achieved after quenching is room temperature (30°C) but the Martensite finish temperature for tool steel is around −50°C and hence there is always retained Austenite present in the material [3–5].

 It is a widely established fact that a long exposure time of a material to very low temperature triggers some definite changes. This caused the tool manufacturers to start exploring the true potential of "Cold Treatment", that is a process in which a material is exposed to a temperature lower than the surroundings.

#### **1.1 Cryogenic treatment**

 Cryogenic treatment is an auxiliary process to the regular heat treatment process. After performing the regular heat treatment process on the steel, it's time for it to undergo cryogenic treatment. The first step of the cryogenic treatment process is to gradually cool down the steel that is at room temperature to a temperature of 77 K (boiling point of liquid nitrogen) at a particular cooling rate. Gradual cooling of the material is done because if it is directly exposed to liquid nitrogen than the material will experience thermal shock and it will make the steel brittle. After achieving the temperature of 77 K, the steel is kept at that temperature for a certain period of time so that the changes that are occurring during the process are throughout the material and uniform properties are obtained by doing so. The next step is to bring the material back to room temperature at a specific heating rate. After this heating process, the material is passed through tempering cycles to regain the lost ductility and to relieve stresses.

### **2. Experiment**

The experiment was preceeded by designing and manufacturing the setup suitable for performing cryogenic treatment. The setup was designed taking into consideration the thermal and mechanical aspects during its functioning. **Figure 1**  shows the actual view of the setup.

**Figure 1.**  *Arrangement of the cryogenic treatment setup.* 

#### **2.1 Working of the setup**

The flow of liquid nitrogen takes place from the Dewar to the copper coils in the setup with the help of an extraction pump. Before pumping liquid nitrogen into the setup, a vacuum i created in the annular space provided in the setup to reduce heat transfer by conduction and convection. The tool is kept in a beaker that is surrounded by copper coils. As liquid nitrogen flows through the copper coils, it gradually reduces the tool's temperature to a desired temperature. A thermocol box is provided at the exit of the copper coil for storing the liquid nitrogen. After attaining a stable temperature, a liquid nitrogen bath is provided for soaking the tool for a duration of 24 hours. A tempering process is carried out to regain the lost ductility and increase the toughness in the tool. The temperature of the tool is measured with the help of RTD, which is connected to the data logger. The data logger takes the temperature reading every 10 seconds.

 After completion of the cryogenic treatment cycle, the tool undergoes various tests that help in validating the benefits of cryogenic treatment. These tests include checking for wear on the tool after a given time, hardness and microstructure examination.

For experimentation purposes, M2 HSS Tool Steel was used. It is one the most widely used tool materials in simple turning operation. The physical characterisation of the tool was measured with the help of a spectrometer, toolmakers microscope, Rockwell hardness tester, and metallurgical microscope.

#### **3. Results and discussion**

#### **3.1 Chemical composition**

The Spark Spectrometer was used to measure the composition of the specimen and also to verify its contents with ASME standards (**Table 1**).


**Table 1.** 

*Chemical composition of M2 HSS tool.* 

#### **3.2 Microstructure examination**

The microstructure examination was performed using standard half inch M2 HSS tool steel. The first step in this process is specimen preparation, which includes polishing the specimen with emery paper and then etching it in a solution of 3% Nital. A metallurgical microscope was used to analyse the microstructure of both the cryogenically treated and untreated specimen. **Figures 2** and **3**, represent the microstructure of both specimens taken at 200X magnification.

 Fine carbide precipitation (black spots) was observed widely in **Figure 2** compared to **Figure 3**, which is a direct correlation to the improvement in dimensional stability as well as wear resistance of the tool [6, 7].

*Performance Evaluation of Deep Cryogenic Treatment on M2 HSS Tool Steel DOI: http://dx.doi.org/10.5772/intechopen.81083* 

**Figure 2. Figure 3.** *Microstructure of cryogenically treated tool. Untreated tool.* 

#### **3.3 Tool wear and tool life measurement**

The measurement of tool wear of both the specimens was carried out using identical working conditions during turning operations on a lathe machine (**Table 2**).

 The following parameters for speed, feed and depth of cut were maintained for both the untreated and cryogenically treated tools. Before the experiment, the length of wear lend was measured with the help of a toolmakers microscope (**Table 3**).


#### **Table 2.**

*Tool geometry for experimentation purpose.* 


**Table 3.** 

*Machining parameters set on the lathe machine during turning operation.* 

After 500 seconds of machining, tool wear lend was measured with the help of toolmakers microscope (**Figure 4**).

The tool wear experiment revealed performance improvements in the cryogenically treated tool when compared to the one not treated. The cryogenically treated tool showed less wear during the course of the experiment, nearly 150 μm less than its counterpart (**Table 4**) (**Figure 5**).

The tool life experiment was carried out by keeping the same machining parameters constant for both the tools (**Table 5**).

The percentage increase in tool life after cryogenic treatment can be calculated by:

Percentage increase in tool life = (1−2)/2 = 48.07%.

*Proceedings of the 4th International Conference on Innovations in Automation...* 

#### **Figure 4.**

*Wear measurement of the specimens using a toolmakers microscope.* 


#### **Table 4.**

*Wear measurement after specific intervals of time.* 

#### **Figure 5.**

*Performance analysis of cryogenically treated tool and untreated tool.* 


#### **Table 5.**

*Tool life values for both the tools.* 

#### **3.4 Hardness measurement**

The hardness of the specimens was measured using the Rockwell hardness tester.

Values in **Table 6** reveal that there is no change in hardness values of the tools [7].

*Performance Evaluation of Deep Cryogenic Treatment on M2 HSS Tool Steel DOI: http://dx.doi.org/10.5772/intechopen.81083* 


**Table 6.**  *Hardness values in HRC.* 
