Hardfacing and Surfacing by the Numbers

About this book

Some parts are made to be worn away. Crusher hammers, loader bucket edges, excavator teeth, auger flights, tillage points, conveyor chutes and pump impellers all lose metal to the rock, soil, grain or slurry they handle, and replacing them is a large part of the running cost of mines, quarries, farms and processing plants. Weld deposits can rebuild worn parts to size and cover them with surfaces far more wear-resistant than the original steel, often at a fraction of the cost of new parts. Done well, hardfacing can multiply a part’s life; done badly, it cracks, spalls or wears no better than plain steel.

This book is for maintenance engineers, repair welders and plant managers who rebuild and protect wearing parts. It assumes a working knowledge of welding and basic materials. It explains, with numbers, how surfaces wear, how building up differs from hardfacing, how the base metal dilutes the first layers of a deposit, how to choose among the families of hardfacing alloys, how to avoid cracking and spalling, how to place deposits where they do the most good, and how to judge hardfacing by what it costs per tonne of material handled.

Early manuals as a starting point

The railway shops of the early twentieth century rebuilt worn parts on a large scale. George W. Cravens’s treatise of 1921 listed driving wheels built up on their treads instead of being turned down, and axle journals built up to their original size. E. Wanamaker and H. R. Pennington, electrical engineers with the Chicago, Rock Island and Pacific Railroad, described building up wheel flanges, journal boxes and shafts by arc welding in the same year. H. A. Hornor’s Spot and Arc Welding of 1920 taught padding, the laying of parallel beads to build up worn parts, and reproduced a railway’s electrode specification that included a manganese steel electrode for hard-wearing surfaces. Hornor also quoted a railway’s costs for filling worn spots by arc welding against older methods, a saving of the kind that made the practice spread.

Their methods are the ancestors of modern hardfacing. Each chapter takes up one of their themes and works it through for a modern wearing part.

The running case

This is an illustration. A maintenance workshop serves a regional quarry and the farms around it. It rebuilds and hardfaces the quarry’s crusher hammers and loader bucket edges, the farms’ tillage points and auger flights, and the conveyor chutes and pump parts of a nearby grain terminal. Its manager wants to choose deposits and processes on evidence, cut the cost per tonne of the quarry’s wearing parts, and stop the cracking and spalling that have spoiled some jobs.

The workshop, its customers, parts, costs and results are invented for teaching. They are not KEVOS® work. Hardfacing of parts whose failure could injure people, and of parts under warranty, must follow the equipment maker’s instructions.

How to use the book

Chapter 1 covers how surfaces wear. Chapter 2 separates building up from hardfacing, and Chapter 3 calculates dilution. Chapter 4 surveys hardfacing alloys, and Chapters 5 and 6 deal with manganese steel and carbide deposits. Chapter 7 compares processes, and Chapter 8 preheat, buffer layers and cracking. Chapter 9 covers deposit patterns and thickness, and Chapter 10 the economics and the wear record. Every chapter ends with an exercise and its worked answer.

Limits and professional responsibility

This book is educational. Hardfacing alloys, preheats and procedures must follow the filler makers’ data and the equipment makers’ instructions. Some hardfacing consumables contain chromium, manganese, nickel, cobalt or other elements whose fume is hazardous, and fume controls must follow the safety data sheets and the work health and safety regulator’s requirements, including Safe Work Australia’s model code of practice for welding processes as adopted in each jurisdiction. Parts whose failure could injure people are repaired only as their makers permit.

An original treatment

READOS covers rebuilding worn shafts and gears in Machine Part Repairs by the Numbers, and the weldability of steels in Weldability by the Numbers; this book links to them rather than repeating them. The explanations, the workshop case, the calculations, figures and exercises are original KEVOS® writing. The historical sources are public-domain works in the United States; their methods are summarised in new words, and none of their text, tables or illustrations is reproduced.