1.1 Stainless Steels and Super Alloys as Difficult-to-Cut Materials
In recent decades, engineering materials have greatly developed. At the same time, the cutting speed and the material removal rate (MRR) in machining such materials using traditional methods such as turning, milling, drilling, grinding, and so on, has been going down. In many cases, it has been challenging to machine these materials such as stainless steels, refractory metals and alloys, Ti-alloys, super alloys, carbides, ceramics, composites, and even diamond, using traditional methods. It is no longer possible to find tool materials that are sufficiently hard to cut such materials.
To meet these challenges, new processes with advanced methodology and tooling have needed to be developed. These are the nontraditional processes, which are capable of machining a wide spectrum of these difficult-to-cut materials irrespective of their hardness. The increasing use of ceramics, high strength polymers, and composites will also necessitate the use of nontraditional methods of machining. In addition, grinding will be applied to a greater extent than in the past, with greater attention to creep feed grinding (CFG), and the use of polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) [1].
The question now is why both stainless steels and super alloys, as difficult-to-cut materials, have both been selected in regard to their machinability in this book? The reasons are as follows:
- There are diverse and important industrial applications necessitating the use of special materials and alloys, characterized by high strength, high temperature strength, and high corrosion, and oxidation resistance.
- There is difficulty associated with machining both materials, especially as they comprise dozens of grades of different machining characteristics.
- Both materials are characterized by low thermal conductivity, high coefficient of thermal expansion, high ductility, and high work-hardening rate, making their machining a tedious task. Their low thermal conductivity leads to an increase in tool temperature, and consequently reduces the tool life. The high work-hardening rate and low thermal conductivity affect chip formation leading to segmented chips. Also, the high coefficient of thermal expansion of these alloys leads to serious difficulties in maintaining machining tolerances.
- The high ductility favors development of built-up edge (BUE) on the tool, thus destroying surface finish, and promoting vibration and chatter.
The basic issue in achieving optimum machining of stainless steels and super alloys is to select adequate cutting speed and correct tool for each work material. In order to realize this objective, a good understanding of the effect of cutting speed on mechanical and thermal properties of the work material and cutting tool should be considered. Some other techniques which are used to enhance the machinability of stainless steels and super alloys will be presented in relevant chapters of this book.
1.1.1 Historical Background of Stainless Steels and Super Alloys
1.1.1.1 Stainless Steels
Stainless steels (SSs) were introduced at the beginning of the twentieth century as a result of pioneering work in England, Germany, and France. However, the development had started several decades before. In 1821, the French Berthier found that iron-chromium alloy was resistant to some acids. Others studied the effects of Cr in an iron matrix, but using a low percentage of Cr. In 1875, another Frenchman, Brustline recognized the importance of carbon levels in addition to Cr.
In 1904, Leon Guilet published a research on martensitic and ferritic SS-alloys with composition that today would be known as 410, 442, 446, and 440 C. In 1906, he also published a detailed study of an Fe-Ni-Cr austenitic alloy; that was equivalent to the 300 series of SS. In Germany, in 1908, Monnartz and Borchers found evidence of the relationship between a minimum level of Cr (10.5%) on corrosion resistance as well as the importance of low carbon content and the role of Mo in increasing corrosion resistance to chlorides.
Harry Brearley of Sheffield was generally accredited as the initiator of the industrial era of SS. He was trying to develop a new material for barrels for heavy guns that would be resistant to abrasive wear. He noted that materials with high Cr-contents did not take an etch. This discovery had led to the patent of a steel with 9â16% Cr and less than 0.7% C; the first stainless steel had been born. Most of his work was on stainless 430, patented in 1919. The first product was the table cutlery that is still used today.
Parallel with the work in England and Germany, F.M. Becket was working in Niagara Falls, to find a cheap and scaling-resistant material for furnaces that run up to 1000 °C. He found that at least 20% Cr was necessary to achieve resistance to oxidation or scaling. That was the first development of heat-resistant steels.
The worldâs first free-machining stainless, invented by Frank Pahlmer (1928) [2], was a straight-grade with sulfur (0.15%S). It was the forerunner of todayâs martensitic 416 stainless. Sulfur and phosphorous were both added to make the austenitic 303 stainless which is the first free-machining Cr-Ni grade in the early 1930s. Selenium (Se) additions instead of sulfur have been favored in the States.
1.1.1.2 Super Alloys
Stainless steels served as a starting point for the satisfaction of high temperature engineering requirements. Moreover, they were soon found to be limited in their strength capabilities. The metallurgists responded to the increased needs by making what might be termed super alloys (SAs) of stainless varieties. Of course, it was long before the hyphen was dropped and the improved iron-base materials became known as one type of super alloy.
The term super alloy was first coined shortly after the second World War to describe a group of alloys developed for use in turbo-superchargers and aircraft turbine engines that required high performance at elevated temperatures. For more than six decades now, super alloys have provided the most reliable and cost-effective means of achieving high operating temperatures and stress conditions in aircraft, and also land gas turbines. As we move towards the third decade of the twenty-first century, super alloys seem to be extending their useful temperature range along with their excellent inherent characteristics. The development of these materials continues to this day with optimization of chemical composition and production methods. This will lead to the development of a new class of material tailored to meet the need for better mechanical properties at elevated temperatures.
Although patents for Al- and Ti-additions to Nichrome type alloys were issued in the 1920s, the super alloy industry emerged with the adoption of Co-base super alloys (Haynes, Stellite 31) to satisfy the increasing demands of higher temperature strength of aircraft engines. Some Ni-Cr-alloys (Inconels and Nimonics), based more or less on toaster wire and developed in the first decade of the twentieth century, were also available for engineering applications. So the race was on to make superior metal alloys available for the insatiable thirst of the designer for higher temperature strength capability. The race still continues.
1.1.2 Industrial Applications of Stainless Steels and Super Alloys
1.1.2.1 Stainless Steels
The average person has no idea what stainless steel is, but it is all around us. Most of us use stainless steel table ware and wear a wristwatch with a stainless steel case. There are stainless steel racks in refrigerators and ovens and there are stainless steel toasters, tea kettles, and even kitchen sinks. Ca...
