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Thermit Welding by the Numbers
Chemistry, heat, charge, mould, preheat and shrinkage for aluminothermic welds, from Goldschmidt's 1898 process to modern rail and heavy-section work
Understand aluminothermic welding from first principles: the reaction and its heat, sizing the charge, moulds, preheat, shrinkage, alloying, rail welding, testing and cost.
Start readingAbout this book
Thermit welding makes its own steel. A mixture of aluminium powder and iron oxide is ignited in a crucible; the aluminium takes the oxygen from the iron in a violent reaction that leaves white-hot liquid iron under a layer of molten aluminium oxide slag; and the iron is run into a mould around the joint, where it melts into the parts and freezes into a weld. No electricity, no gas supply and no welding machine are needed, which is why the process has joined rails in the field for more than a century and why it was once used for repairing ship stern frames, mill housings and crankshafts too large to move.
This book is for maintenance and reliability engineers, welding supervisors, rail and heavy-plant engineers and students who need to understand aluminothermic welding well enough to specify, assess or supervise it, and to judge when it is the right repair. It assumes basic chemistry, physics and algebra in SI units. It does not teach anyone to carry out a thermit weld. That requires the supplier’s training and accreditation, approved procedures and the specific safety controls the process demands.
Two early books as a starting point
Hans Goldschmidt of Essen discovered how to ignite a cold mixture of aluminium and iron oxide around 1895 or 1896, patented the reduction of metals by aluminium in 1897 and used it to weld iron about 1898. By 1910, when Richard N. Hart published Welding: Theory, Practice, Apparatus and Tests, the Thermit process was established for rail joints and heavy repairs. Ethan Viall, editor of American Machinist, gave it a large section of his Gas Torch and Thermit Welding in 1921.
Both books are full of numbers: the proportions of the mixture, the calculated and measured temperatures, the rule for how much thermit a mould needs, the size of the collar, the additions that strengthen the steel and the results of tests. Most of those numbers can be checked with modern chemistry and physics, and doing so is a good way to understand the process. Each chapter takes one of their themes, checks it and works it through a modern case.
The running case
This is an illustration. A quarry’s primary jaw crusher has a cast steel side frame with a crack right through one of its tension members, a solid section 200 × 150 mm. The crusher is large, the frame is expensive and slow to replace, and the quarry’s maintenance engineer must choose a repair. One option, put forward by a specialist contractor, is a thermit fusion weld across the cracked member. The quarry also has a rail siding whose rails were joined by aluminothermic welds, and one of those welds has failed.
Through the book, the engineer works out the chemistry, the charge, the mould, the preheat, the shrinkage allowance, the weld metal, the testing and the cost, and compares them with arc welding and replacement. The quarry, the crusher, the dimensions, quantities and costs are invented for teaching. They are not KEVOS® work, and nothing in the case is a procedure for carrying out a thermit weld.
How to use the book
Chapters 1 to 3 cover the reaction: its chemistry, its heat and temperature, and the ignition, moisture and other hazards that come with it. Chapters 4 to 7 cover the weld: sizing the charge, moulds and risers, preheating, and shrinkage. Chapter 8 deals with the weld metal and its additions, Chapter 9 with rail welding today, and Chapter 10 with testing, cost and the choice of repair. Every chapter ends with an exercise and its worked answer.
Limits and professional responsibility
This book is educational. Thermit welding involves molten metal at well over 2,000 °C, violent reactions with moisture, fire, fumes and heavy lifting. It must be carried out only by trained and accredited people, using the supplier’s approved materials and procedures, under the controls the work health and safety regulator, the asset owner and, for rail, the rail safety framework require. Rail welds in Australia are qualified under AS 1085.20 and the rail infrastructure manager’s standards. Repairs to load-bearing machinery must be designed, approved and inspected by competent engineers. The 1910 and 1921 methods are described as history, not practice.
An original treatment
READOS covers steel repair in Weld Repairs That Hold, how metals behave in welding in Weldability by the Numbers and electric arc welding in Arc Welding by the Numbers. This book covers the aluminothermic process. The explanations, the crusher case, the calculations, figures and exercises are original KEVOS® writing. The historical books are public-domain works in the United States; their facts are summarised, checked and corrected in new words, and none of their text, tables or illustrations is reproduced.
Who this book is for
Maintenance and reliability engineers, welding supervisors, rail and heavy-plant engineers and students who need to understand, specify, assess or supervise aluminothermic welds and judge when they are the right repair.
What you will learn
- Balance the aluminothermic reaction, calculate the iron it yields and estimate the heat it releases and the temperature it can reach.
- Size a thermit charge from the mould volume, and explain the purpose of the collar, runner and riser.
- Estimate the heat needed to preheat the parts and the allowance needed for shrinkage.
- Explain how additions alter the weld metal and why modern rail portions are matched to the rail steel.
- Plan inspection and testing of a thermit weld, and compare its cost and risk with other repair methods.
Inside this book
Chapters
- 01The aluminothermic reactionBalance the reaction between aluminium and iron oxide, calculate how much iron and slag a charge yields, and check the 1921 proportions with modern atomic masses.
- 02Heat and temperature of the reactionCalculate the heat a thermit charge releases from enthalpies of formation, estimate the temperature it can reach, and compare 1921 calculations and measurements with modern figures.
- 03Ignition, moisture and safetyWhy thermit is hard to ignite but violent once started, how a few millilitres of water become hundreds of litres of steam, and how a thermit job is planned around its hazards.
- 04Sizing the chargeTurn the 1910 rule of eighteen ounces per cubic inch into SI, size the crusher-frame charge from the mould volume or a wax pattern, and see what an undersized charge does.
- 05Moulds, risers and the collarDesign the mould around a thermit weld: the collar that reinforces it, the runner that fills it from the bottom, and a riser sized so the weld freezes before its feed does.
- 06PreheatingWork out why the parts are heated to red heat before the pour, how much heat and gas that takes for the crusher frame, and what an unevenly heated joint does to fusion.
- 07Shrinkage, gap and distortionEstimate how much a thermit weld and its heated parts shorten as they cool, why that matters in a closed frame, and how the parts are spread or the opposite member heated to compensate.
- 08Alloying the weld metalSee what plain thermit steel is made of, how punchings, manganese, nickel and modern alloy additions change it, and why a weld's composition must suit the steel it joins.
- 09Rail welding todayWhy rails are still joined by thermit in the field, the forces a continuous rail carries, how rail welds are qualified in Australia, and how the quarry audited its siding welds.
- 10Testing, cost and choosing thermitPlan the inspection of a finished thermit weld, compare its cost and risk with an arc weld repair and a replacement part, and record the decision so others can check it.
References and source notes
The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 25 October 2026.
- Ethan Viall (McGraw-Hill, New York). Gas Torch and Thermit Welding (1921). Historical source for the history of the Thermit process, the 1921 reaction and proportions, the theoretical and measured temperatures, ignition and the need to keep thermit dry, the plastic and fusion methods, preheating, the varieties of thermit and the composition and strength of thermit steel. Summarised and discussed in new words; no text, tables or illustrations are reproduced, and its practices are not modern instructions.
- Richard N. Hart (McGraw-Hill, New York). Welding: Theory, Practice, Apparatus and Tests (electric, thermit and hot-flame processes) (1910). Historical source for the 1910 rule for sizing a thermit charge from mould volume or from the weight of a wax pattern, collar proportions, crucible practice, additions of steel punchings, manganese and nickel, and reported repair and test examples. Summarised and discussed in new words.
- George W. Cravens (American Technical Society, Chicago). Welding: a practical treatise on the applications of electric, gas and thermit welding (1921 (copyright 1920)). Historical survey of thermit welding consulted for context. Summarised in new words.
- L. A. Groth (D. Van Nostrand, New York). Welding and Cutting Metals by Aid of Gases or Electricity (1909). Historical survey of early aluminothermic welding consulted for context. Summarised in new words.
- TWI. What is thermite welding (CAD / exothermic welding)? (FAQ) (technical FAQ). Source for the modern description of exothermic welding of copper conductors and rails, reaction temperatures of around 2,000 °C, preheating rail ends, alloying additions in rail thermite, ignition with a booster, the hazards of smoke and ejection, and the use of rail thermite welding in the United States since at least 1935.
- National Institute of Standards and Technology. NIST Chemistry WebBook, standard enthalpies of formation of aluminium oxide, iron(III) oxide and iron(II,III) oxide (online reference). Source of the standard enthalpies of formation used in Chapter 2 to calculate the heat of the aluminothermic reaction: Al₂O₃ −1,675.7 kJ/mol, Fe₂O₃ −824.2 kJ/mol and Fe₃O₄ −1,118.4 kJ/mol.
- Standards Australia; Rail Industry Safety and Standards Board (RISSB). AS 1085.20, Railway track material, Part 20: Welding of steel rail (2020 edition). Named in Chapters 9 and 10 by number and scope: requirements for qualifying flash butt and aluminothermic welds in steel rail and for railhead repair. No content or values are reproduced.
- Safe Work Australia. Model Code of Practice: Welding processes (2020). Cited for general welding hazards, fumes, burns and fire, which apply to thermit welding along with its own specific hazards. Check which version applies in your jurisdiction.
