Journal of Engineering and Technology

ARTICLE
Year
: 2011  |  Volume : 1  |  Issue : 2  |  Page : 88--93

Effects of Cryogenic Treatment on High-speed Steel Tools


Lakhwinder Pal Singh, Jagtar Singh 
 Department of Mechanical Engineering, SLIET, Longowal, Sangrur, Punjab, India

Correspondence Address:
Lakhwinder Pal Singh
Department of Mechanical Engineering, SLIET, Longowal, Sangrur, Punjab
India

Abstract

High-speed steel (HSS) tools are the most commonly used tools in small and medium-scale industry. So far, a few studies have been carried out pertaining to the life of HSS single point cutting tool. Studies on cryogenically treated (CT) cutting tools show microstructural changes in the material that can influence the life of the tools significantly. This paper primarily reports performance of CT HSS tools as compared to untreated (UT) HSS tools. The results show that CT HSS tools exhibit better performance based on tool wear, surface roughness of the work specimen, and power consumption during operation than the UT HSS tools.



How to cite this article:
Singh LP, Singh J. Effects of Cryogenic Treatment on High-speed Steel Tools.J Eng Technol 2011;1:88-93


How to cite this URL:
Singh LP, Singh J. Effects of Cryogenic Treatment on High-speed Steel Tools. J Eng Technol [serial online] 2011 [cited 2019 Dec 6 ];1:88-93
Available from: http://www.onlinejet.net/text.asp?2011/1/2/88/86640


Full Text

 1. Introduction



The commonly used cutting tool material in conventional machine tools is high-speed steel (HSS). As the technology has been rapidly advancing, newer cutting tool materials such as cemented carbides, cermets and ceramics are needed to machine many difficult to machine materials at higher cutting speeds and metal removal rates with performance reliability. In recent years, increased interest in the effects of low temperature on tool and die materials, particularly HSS tools, has been demonstrated. Over the past few years, there has been an increase in the application of cryogenic treatment to different materials. Research has shown that cryogenic treatment increases product life, and in most cases provides additional qualities to the product, such as stress relieving, toughness, etc.

In the area of cutting tool, which includes HSS and medium carbon steels, cryogenic treatment can double the service life of tools.

 2. Literature Survey



Mohan Lal et al. [1] studied the improvement in wear resistance and the significance of treatment parameters in different tool and die materials. They found that cryogenic treatment imparts nearly 110% improvement in tool life. Cohen et al. [2] proved that the power consumption of cryogenically treated (CT) HSS tools is less when compared to the untreated HSS tools. Cryogenic treatment of tool steels is a proven technology to increase the wear resistance and extend intervals between component replacements for blades, bits, machining mills, etc., and hence improves surface quality of the machined parts. Combining optimized lubrication, correct mechanical configuration and cryogenic treatment of wearing parts results in the maximum performance of lubricated components and can significantly extend the component life. Reliability of operating is influenced by five factors: Component design, manufacture, specifications, installation and maintenance. Each of these stages can be influenced by separate individuals or teams, but ultimately the responsibility for performance of the assembled system falls on the plant maintenance team. It has been said that machines do not die, people kill them. In many cases this is true, and many of the contributing factors to premature failure can be controlled by the end user. However, if the equipment has been properly installed and maintained, exerting influence on the other factors may be difficult or impossible for the end user. Cryogenic treatment has been claimed to improve the wear resistance of steels and has been implemented in cutting tools since long. Although it has been confirmed that cryogenic treatment improves the wear resistance and tool life, the process has not been standardized, with the results being inconsistent, varying from researcher to researcher [3] .

The literature published in this regards reports that cryogenic treatment facilitates the formation of carbon clustering and increases the carbide density in the subsequent heat treatment, which further improves the surface quality and wear resistance of steels. Cryogenic treatment is a sub-zero thermal treatment generally given to ferrous tool materials. In this treatment, the tool materials are subjected to temperature below −196°C (−320°F) for 20-60 hours in well-insulated chambers and liquid nitrogen (LN 2 ). Studies on CT HSS tools show microstructural changes in the material that can influence tool lives and productivity significantly [4] .

The life of cutting tool is affected by factors like cutting speed, feed and depth of cut, tool material, heat treatment of the tool, work material and nature of cutting. The main characteristics of a good cutting tool material are its hot hardness, wear resistance, impact resistance, abrasion resistance, heat conductivity, strength, etc. What is important to tool life is the likely changes in these characteristics at high temperature because the metal cutting process is always associated with generation of high amount of heat, and hence high temperatures. Cutting speed has the maximum effect on tool life, followed by feed rate and depth of cut. All these factors contribute to the rise of temperature. That is why it is always said that an ideal tool material is the one which will remove the largest volume of work material at all speeds. It is, however, not possible to get a truly ideal tool material. The tool material which can withstand maximum cutting temperature without losing its principal mechanical properties (especially hot hardness) and geometry will ensure maximum tool life, and hence will give the most efficient cutting of metal [5] .

During the cutting operation, cutting tool is subjected to static and dynamic forces, high temperatures, wear and abrasion [6] . Yu and Bhole [7] concluded that abrasive wear resistance is dependent on the tribological system; it depends not only on the intrinsic conditions of the material, but also on the soil conditions. The influential factors are: the chemical composition, production history, mechanical properties and microstructure of material; the particle shape, size; the soil strength, density and moisture; the relative velocity and impact angle between soil and the tool. Thus, there is no simple relationship between the abrasive wear resistance and the common mechanical properties.

Singh [8] conducted experimentation on the effect of cryogenic treatment on machining characteristics of titanium alloy (Ti-6A1-4V). In his experimentation, he predicted the best rpm range for conventional milling of titanium alloy (Ti-6A1-4V) using HSS tool material. The specimen was a cryogenic treated cylindrical rod for which a cryogenic treated HSS end mill was used to generate a cavity. The mechanical properties, namely, surface roughness, surface hardness, metal removal rate, and tool wear rate of the machined surface, were observed to find out the best range of machining characteristics. The results indicated that best machining range is between 300 and 500 rpm, surface roughness improves by 47.91%, surface hardness increases by 2.25%, material removal rate increases by 4.38% and the tool wear rate decreases by 52.9%.

Grewal [9] studied the effect of cryogenic treatment of the wire on machining performance of wire cut Electric Discharge Machine. He found in his study that metallic materials having high mechanical strength generally show a low electric conductivity, whereas those having a high electric conductivity generally show a low mechanical strength. With the help of cryogenic treatment of wire, current carrying capacity of the wire can be increased. It is also expected that the cryogenic treated wire would have less chances of breakage during machining as compared to untreated wire because of increase in its toughness.

Vadivel et al. [10] reported that cryogenic treatment has been acknowledged in several researches as a means of extending the tool life of many cutting tools. Studies on CT cutting tools show microstructural changes in the material that can influence the life of the tools significantly. Tungsten carbide cutting tools are now commonly used in the industry. So far, only a few detailed studies have been carried out pertaining to the cryogenic treatment of carbides. This paper primarily reports and analyzes various performances of CT-coated carbide inserts and untreated (UT) coated carbide inserts in turning of nodular cast iron. From the results, it can be seen that CT-coated carbide inserts exhibit better performance based on the surface roughness of the work specimen, power consumption, and flank wear than the UT ones. The scanning electron microscope analysis is carried out for the worn out CT- and UT-coated carbide inserts to predict the wear resistance.

Cajner et al. [11] reported that the advantages of deep cryogenic treatment over standard heat treatment of HSS for the purpose of obtaining better exploitation properties are quoted in an increasing number of scientific papers in the literature. This paper deals with the most important improvements of HSS properties, achieved by using deep cryogenic treatment. The effect of deep cryogenic treatment on impact and fracture toughness, erosion wear resistance and the material microstructure has been tested on test pieces made of PM S390 MC HSS. A set of test pieces was heat treated by conventional methods (hardened and three times high temperature tempered).

Podgornik et al. [12] investigated the influence of deep cryogenic treatment parameters (treatment time and temperature) on the tribological performance of powder-metallurgy (P/M) HSS. Special emphasis was laid on abrasive wear resistance and resistance to galling under dry sliding conditions. Results were evaluated in terms of HSS wear volume, coefficient of friction under reciprocating sliding, friction variation with load, critical load for galling initiation, and stainless steel transfer layer formation. Tribological tests indicate that deep cryogenic treatment contributes to improved abrasive wear resistance and better galling properties of P/M HSS. However, austenitizing temperature is a very important parameter which defines the level of possible improvement.

The science and technology of producing a low temperature environment is referred to as cryogenics. The word cryogenics has its origin in the Greek language where "cryos" means frost or cold and "gen" means generate. Cryogenic processing has been around for many years but is truly in its infancy when compared to heat-treating. For centuries the Swiss would take advantage of the extremely low temperatures of the Alps (Fridge Regions) to improve the behavior of their steels. They would allow the steel to remain in the frigid regions of the Alps for long periods of time to improve its quality. Essentially, this was a crude aging process accelerated by the very low temperatures. What we now understand to have happened was the reduction of the retained austenite and the increase in martensite. By performing this once secret process the Swiss obtained the reputation for producing a superior grade of steel [13] .

The process of experimentation and understanding of the cryogenic treatment of steels really got under way during World War II. It was under the direction of Clarence Zener who would later go on to develop the Zener diode. At that time there were no computer controls so the steel tooling would be immersed in liquid Nitrogen for a brief period of time, allowed to warm up, then placed into service. This method was crude and uncontrolled. Many of the tools would chip and break immediately upon use because the immersion process would create a very high thermal gradient in the tool and this would produce micro-cracks in the body. It was also later learned that the cryo-treatment would convert the retained austenite into un-tempered martensite. But the tools that would not break would experience a greatly enhanced service life. In the 1960's cryogenic processors would use multi-stage mechanical coolers along with insulated 'cold-boxes' to gently remove the latent heat from tooling thereby achieving a much slower cooling rate, concurrent with longer wear lives [14] .

A typical soak segment will hold the temperature at −300 o F for some period of time, typically eight to forty hours. During the soak segment of the process the temperature is maintained at a low temperature. Although things are changing within the crystal structure of the metal at this temperature, these changes are relatively slow and need time to occur. One of the changes is the precipitation of fine carbides. It is found that soaking time in the process also provides time for the crystal structure to react to the low temperature and for energy to leave the crystal structure. By keeping the part at a low temperature for a long period of time, it is found that some of the energy go out of the lattice and making a more perfect and therefore stronger crystal structure [15] .

Jo J. Braz et al. [16] stated that Cryogenic cooling is an efficient way of maintaining the temperature at the cutting interface well below the softening temperature of the cutting tool material. This technology is exploited mainly in the grinding industry because of the high specific energy requirements which results in high grinding zone temperature which if not properly controlled will lead to surface damage. The principle of cryogenic cooling in grinding involves directing a jet of liquefied gases under pressure into the grinding zone. Commonly used coolant is liquid nitrogen (LN2) because of its low cost and the fact that it does no harm to the environment. When turning a high strength reaction bonded silicon nitride (RBSN) with cubic boron nitride (CBN) cutting tool, it was found that the maximum temperature generated at the cutting interface was only 829 o C as against 1153 o C in dry machining. This temperature is far below the softening temperature (1500 o C) of CBN tool material, hence the improved tool performance obtained with the LN2 cooling system. It has also been shown that in turning applications directing the LN2 coolant jet directly to the cutting interface at the rake face and the secondary flank face simultaneously improve tool performance significantly. In cryogenic machining high cutting and thrust forces are generated than in conventional and fluid cooling or dry machining applications. This anomaly is attributed to the fact that sub zero temperatures has the consequence of increasing hardness and strength of the work material, hence higher forces are generated with cryogenic cooling. Tool wear rates when machining titanium alloy Ti-6Al-4V with cemented carbide using LN2 and under conventional cooling at a cutting speed of 132 m min−1 , feed rate of 0.2 mm rev−1 and a depth of cut of 1.0 mm showed a fivefold increase in flank wear for tools subjected to the conventional cooling.

T. Ozel et al. [17] stated that in machining of parts, surface quality is one of the most specified customer requirements. Major indication of surface quality on machined parts is surface roughness. Finish hard turning using CBN tools allows manufacturers to simplify their processes and still achieve the desired surface roughness. There are various machining parameters which have an effect on the surface roughness, but those effects have not been adequately quantified. In order for manufacturers to maximize their gains from utilizing finish hard turning, accurate predictive models for surface roughness and tool wear must be constructed. Neural network modeling is utilized to predict surface roughness and tool flank wear over the machining time for variety of cutting conditions in finish hard turning. Regression models are also developed in order to capture process specific parameters. A set of sparse experimental data for finish turning of hardened AISI 52100 steel obtained from literature and the experimental data obtained from performed experiments in finish turning of hardened AISI H-13 steel have been utilized. The data sets from measured surface roughness and tool flank wear were employed to train the neural network models. Trained neural network models were used in predicting surface roughness and tool flank wear for other cutting conditions. A comparison of neural network models with regression models is also carried out. Predictive neural network models are found to be capable of better predictions for surface roughness and tool flank wear within the range that they had been trained. Predictive neural network modeling is also extended to predict tool wear and surface roughness patterns seen in finish hard turning processes. Decrease in the feed rate resulted in better surface roughness but slightly faster tool wear development, and increasing cutting capacity.

 3. Experimental Work



The experimental work was carried out in the workshop of L. R. Institute of Engineering and Technology, Solan. The turning operation was carried out on the work specimen using cryogenically treated and untreated HSS tools. The surface roughness of work specimen, weight loss, flank wear of cutting tools and power consumption were predicted. Micrographs of the UT and CT HSS tools were also obtained. Both the cutting tool blanks (UT and CT HSS) are commercially available, made by Miranda Tools Ltd.

So, the tool blanks, namely, T42-S-400 (UT) and T42-S-500 (CT), were purchased from the market. The tools were prepared as per desired tool geometry. The desired angles and nose radius were cut from the Small Scale Industry Institute, Ludhiana. Brief experimental conditions, machine tool and equipment specifications are given in [Table 1]. This study will further be extended to optimize these parameters by applying some suitable Design of Experiments (DOE) technique like Taguchi method.{Table 1}

 4. Methodology



The various steps involved in the estimation of performance of cryogenically treated HSS tools are as follows:

Step 1: Nose radius of both the UT as well as CT HSS tools was measured before and after the removal of specific volume of material.

Step 2:

Weight of both the tools was measured before and after the removal of specific volume of material.

Step 3:

Power consumed by the lathe machine was measured using UT as well as CT HSS tools for removing a specific volume of material.

Step 4:

Surface roughness of both the work specimens (one machined by UT and the other by CT HSS tool) was measured after the removal of specific volume of material.

 5. Parameter Measuring Techniques and Equipments



Surface roughness of work specimen was measured by surface roughness tester, model SURF TEST 4; nose radius of both the UT and CT HSS tools was measured using Universal Measuring Microscope Least Count (LC) 0.0001 mm; and microstructure of both the UT and CT HSS tools was taken using Metallurgical Microscope, magnification ×100. All these tests were conducted at Research and Development Centre for Bicycle and Sewing Machine (UNDP-UNIDO Assisted Punjab Govt. Project), Ludhiana, Punjab.

Initial and final weight of the tools was measured with an electronic balance, Citizen Make, LC 0.01 mg. Power consumption was calculated by measuring the current (amperes), voltage (volts) with Digital Tong-type Multimeter, Model MASTECH and total time taken for machining operation. Speed of the work piece was measured with Digital Tachometer, Model DT-2234.

 6. Results and Discussion



The microstructure analysis was carried out to study the microstructure changes in HSS UT and CT tools due to cryogenic treatment. From the micrographs shown in [Figure 1] and [Figure 2], it can be seen that the microstructure of HSS gets more refined, with uniformly distributed fine alloy carbides in tempered martensite, after the cryogenic treatment. The parameters chosen to study the performance of CT HSS tools were change in nose radius and weight loss of cutting tools, power consumption by the lathe machine and surface roughness of the work material after machining. In laboratory tests, wear of both the tools was found to be significantly different. Results for all the above-said responses are shown in [Table 2].{Figure 1}{Figure 2}{Table 2}

The results for wear, i.e. nose radius and weight loss, vary widely. From the analysis, it can be seen that the wear of UT HSS tool is more than that of CT HSS tool. The weight loss is also more in case of UT HSS tool as compared to CT HSS tool. Power consumed by the lathe machine using UT HSS tool was also more than that of CT HSS tool. The surface roughness of work piece machined with UT HSS tool is more than that of CT HSS tool, which shows that the surface machined with CT HSS tool is smoother as compared to UT HSS tool. It can also be seen from the micrographs of both the tools that the microstructure of CT HSS tool is more refined and uniform as compared to UT HSS tool.

 7. Conclusion



The performance of HSS is quite good because of its composition, especially 10% cobalt. After cryogenic treatment, the performance of cryogenically treated tool had been significantly enhanced. From the micrographs shown in [Figure 1] and [Figure 2], it can be seen that the microstructure of HSS gets more refined and the particles are uniformly distributed after the cryogenic treatment. Less reduction in nose radius, lower weight loss, more uniform distribution of metal particles, refined microstructure of CT HSS tool and lower value of surface roughness of the work piece machined with CT HSS tool represent the positive scope of cryogenic treatment on tool and die materials.

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