Non-Traditional and Advanced Machining Technologies

Helmi Youssef, Hassan El-Hofy

• Publisher: CRC Press
• Year: 2020
• Print ISBN: 9780367431341
• eBook ISBN: 9781000097160
• Topic: Technology & Engineering
• Subtopic: Mechanics
• Index: Technology & Engineering

Non-Traditional and Advanced Machining Technologies covers the technologies, machine tools, and operations of non-traditional machining processes and assisted machining technologies. Two separate chapters deal with the machining techniques of difficult-to-cut materials, such as stainless, super alloys, ceramics, and composites. Design for machining, accuracy and surface integrity of machined parts, environment-friendly machine tools and operations, and hexapods are also presented.

The topics covered throughout reflect the rapid and significant advances that have occurred in various areas in machining technologies and are organized and described in such a manner to draw the interest of the reader. The treatments are aimed at motiving and challenging the reader to explore viable solutions to a variety of questions regarding product design and optimum selection of machining operations for a given task.

The book will be useful to professionals, students, and companies in the areas of industrial, manufacturing, mechanical, materials, and production engineering fields.

Unit II

Advanced Machining Technology

5 Machining of DTC Materials (Stainless Steels and Super Alloys) by Traditional and Non-Traditional Methods

5.1 Introduction

Over recent decades, engineering materials have been greatly developed. These materials such as hardened steels, stainless steels, super alloys, carbides, ceramics, and fiber-reinforced composite materials are frequently applied in modern industry. The cutting speed and the material removal rate when machining such materials using traditional methods such as turning, milling, grinding, and so on, tend to fall. Sometimes, it is difficult to machine hard materials to certain shapes using these 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 to be developed. These are the non-traditional 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 non-traditional methods of machining. In addition, grinding will be applied to a greater extent than in the past, with greater attention given to creep feed grinding (CFG), and the use of poly-diamond (PCD) and poly-cubic boron nitride (PCBN) (ASM International, 1989).

5.2 Traditional Machining of Stainless Steels

5.2.1 Types, Characteristics, and Applications of SSs

Stainless steel does not constitute a single well-defined material, but instead, depending on the alloying elements additions, comprises several families of alloys; each generally having its own characteristics, microstructure, alloying elements, and properties, that are suited to a wide range of applications.

The American Iron and Steel Institute (AISI) and the Unified National Standard (UNS) grouped these alloys by chemistry and assigned a three-digit number (AISI) or a five-digit number (UNS) that identifies stainless steels. Accordingly, stainless steels have been subdivided into five families: three basic families (ferritic, martensitic, and austenitic), and two derived families (duplex and precipitation hardened).

Basic (standard) alloys of stainless steels: These comprise the following three categories:

1. 1. Ferritic stainless steels of AISI designations : Series 400 [405-409-430-434-436-442-444-446], Figure 5.1.
2. These have carbon levels below 0.12% (442 and 446 are at 0.2% C) and high Cr content (10.5–27%), and relatively small amounts of other alloying elements. Higher levels of Cr in alloys 442 and 446 promote their corrosion and oxidation resistance. Mo is added to 434 and 444 to improve corrosion resistance particularly in chloride-containing solution.
3. These grades are magnetic and cannot be hardened by heat treatment, however, hardened by cold working, but not to the same extent as austenitic alloys. Ferritic alloys have reduced corrosion resistance compared to austenitic. They are generally not chosen for toughness. In the annealed condition, they have a yield tensile strength (YTS) of 275–350 MPa.
4. The last four alloys in Figure 5.1 are free-machining ferritic alloys since they contain free-machining additives.
5. Since ferritic alloys are the cheapest type of stainless steel, they should be given first consideration when SS alloy is required. Ferritic stainless steels are generally used for non-structural applications such as kitchen and restaurant equipment, automobile trims, heaters, dishwashers, and annealing baskets.
6. 2. Martensitic stainless steels also of AISI designation: Series 400 [403-410-414-416-420-422-431-440], Figure 5.1.
7. Martensitic alloys have a relatively high carbon level (0.15–1.2% C) as compared to ferritic and austenitic grades, and Cr level from 11.5 to 18%. Mo (<1%) can be added to improve mechanical properties and corrosion resistance (UNS-42010). Nickel (<2.5%) can be added for the same reason, AISI (414 and 431).
8. Martensitic alloys are also magnetic. They are not as corrosion resistant as the other two basic classes. In the annealed condition, the yield tensile strength (YTS) is about 275 MPa, and thus these alloys can be moderately hardened by cold working similar to ferritic alloys. However, when hardened and tempered, their YTS increases up to 1900 MPa, depending primarily on carbon content. In the annealed condition, they are machinable. The last seven alloys in Figure 5.1 are free-machining martensitic alloys, since they contain considerable amounts of free-machining additives of S or Se (minimum of 0.15% each).
9. Martensitic alloys cost about 1.5 times as much as ferritic. Martensitic alloys are used for applications such as cutlery, surgical tools, instruments, valves, rivets, screws, hand tools, vegetable choppers, razor blades, riffle barrels, mining machinery, bolts, nuts, and aircraft fittings.
10. 3. Austenitic stainless steels of AISI designation: Of Series 300 [Fe-Cr-Ni], and Series 200 [Fe-Cr-Mn], Figure 5.1.
11. This category contains maximum 0.15% C, a minimum of 16% Cr and sufficient Ni and/or Mn to retain an...

Table of contents

• Cover
• Half-Title
• Title
• Copyright
• Dedication
• Contents
• Preface
• Acknowledgements
• Author Biographies
• List of Symbols
• List of Acronyms
• Unit I Non-Traditional Machining Operations and Non-Traditional Machine Tools
• Unit II Advanced Machining Technology
• Index