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Non-destructive Testing of Thermite Welds

There are four standard techniques for welding railroad tracks: Flash-butt welding, the gas-pressure welding, the enclosed-arc welding, and thermite welding. In this article, railroad the thermite welding process, thermite weld defects and thermite weld non-destructive testing are discussed.

Railroad thermite welding is a high temperature process that exploits chemical heat of reaction as the heat input source. During the welding process, the two ends of the adjoined rails are mounted in a mold to which a mixture of aluminum powder and iron oxide powder (called thermite) is heated, An exothermic process produces liquid metal and molten slag that are injected into the mold. The liquid metal migrates towards the joint gaps and cools to form weld metal. To control the melt temperature and the weld metal chemical composition, different additives and/or alloys may be introduced.

Field thermite welds may contain continuities or gas pockets, aligned usually with the rail transverse plain. Welding processes that induce thermite weld defects include lack of penetration of weld metal between the rail ends, lack of fusion between weld and rail base metal, slag inclusion entrapment, shrinkage or thermal cracking, and fatigue cracking. It is possible for weld metal cracks to propagate into the rail base metal.

Thermite Weld Quality Control Testing

There are two technical references that outline thermite weld quality control testing and non-destructive testing: 1) AREMA Manual Railway Engineering and AWS D15.2, Recommended Practices for the Welding of Rails and Related Rail Components for Use by Rail Vehicles. Some of the quality control tests required are listed below [1]:

Completed thermite weld
Figure 1: Completed thermite weld

Thermite Weld Chemical Analysis

TKS provides chemical analysis of rails to identify the presence and the amounts (by weight percent) of the following elements: Carbon, Manganese, Phosphorous, Sulfur, Silicon, Nickel, Chromium, Molybdenum, Vanadium and Aluminum. Optical Emission Spectroscopy (OES) analysis is performed to determine the chemical composition of the rails and evaluated with respect to the standards specified in AREMA Manual for Railway Engineering Chapter 4. OES is the reference technique for elemental analysis of solid metallic samples and is capable of performing rapid simultaneous analysis of up to 60 elements.

Thermite Welds Slow Bend Test & Bond Integrity

The rail is supported on a 72″ span with the weld joint centered between the supports, and central static load is applied. For vertical loading, dial gages used to record vertical deflection, are located 3″ on either side of the central loading point and on the center of the rail base. For lateral loading, the load is applied at the center of the span through the rail’s neutral axis, and the deflections at rail head and at edge of rail base are recorded by dial gages located 3″ on either side of the loading point. Bond integrity is determined from the fracture faces of the slow bend test samples. AREMA manual stipulates that the bond between two joining rail ends shall not contain no more than one inch diameter discontinuity.

Thermite Weld Hardness Testing

Brinell Hardness Test (prescribed by ASTM E 10) to carry out precise measurement of the diameter of an indentation made with hardened balls of various diameters under various loads is performed. We provide the optics and analysis software (Digital Optical Comparator System) for the required diameter measurement and subsequent calculation of the Brinell hardness in accordance with the ASTM standards.

Thermite Weld Metallographic Tests

TKS utilizes several techniques for preparing material samples for analysis including metallographic preparation for either micro-structural or macro-structural analysis. Macro-etching involves taking a cross-section of the large component, grinding and polishing the surface and etching the piece and then, visually analyze the degree of micro-structural segregation. TKS also provides micro-structural analysis using scanning electron microscope (SEM) and other microscopic techniques.

Thermite Weld Magnetic Particle Testing

TKS uses standard magnetic particle equipment for testing of thermite welds. The inspection procedure follows the recommendations outlined in ASTM E709 – 08 i Standard Guide for Magnetic Particle Testing. The procedure may vary depending on the rail type, profile, surface condition, etc. AC electromagnetic yokes are used.

Ultrasonic Testing of Thermite Welds

In accordance with AWS D15.2, Recommended Practices for the Welding of Rails and Related Rail Components for Use by Rail Vehicles, All thermite welds shall be ultrasonically inspected in accordance with the class of track in which they are installed [2]. Inspection requirements are outlined in 49 CFR 213.237 (Class 5 and lower), 49 CFR 213.339, and 49 CFR 213.341 (Class 6 and higher). Discontinuity Size Allowance & Remedial Action are defined in 49 CFR 213.113 “Defective Weld” and 49 CFR 213.337 “Defective Weld”.

Phased array ultrasonic testing (PAUT) of rail thermite welds
Figure 2: Phased array ultrasonic testing (PAUT) of rail thermite welds

Phased Array Testing of Thermite Welds

TKS specializes in advanced non-destructive ultrasonic testing including but not limited to phased array testing, time-of-flight-diffraction, TFM/FMC, guided wave ultrasonic testing, and acoustic emission testing. Phased array ultrasonic testing (PAUT) is applied new and in-service asset integrity programs. PAUT identifies fabrication and in-service related rail flaws, weld defects, and in-service deterioration mechanisms like corrosion, fatigue cracks and stress corrosion cracking and corrosion fatigue cracks.

Benefits include:

› Faster inspection: Electronic scanning increases inspection efficiency.

› Accurate sizing: 2-D cross-sectional sizing provides accurate flaw heigh and length for load rating calculations.

› Improved probability of detection: PAUT increases POD by maximizing the amount of ultrasonic energy reflected back from flaws of unknown orientation and geometry.

How it Works

› Scan plan is developed pre-inspection to satisfy customer/code requirements.

› Qualified technician calibrates equipment on-site and preforms inspection.

› PAUT transducer and equipment sweep through defined range of angles and focal depth searching for material defects.

› Accurate intensity based images are generated of flaws

› Technical report provided detailed location and sizes of flaws detected


  1. American Railway Engineering and Maintenance-of-Way Association (AREMA), Manual for Railway Engineering (MRE), Chapter 4 – Rail, Specification for the Quality Assurance of Thermite Welding of Rail

  2. AWS D15.2, Recommended Practices for the Welding of Rails and Related Rail Components for Use by Rail Vehicles, Chapter 8, 2013.

  3. 49 CFR 213.237 – Track Safety Standards, Inspection of rail

  4. CFR 49 213.113 - eCFR __ 49 CFR Part 213 Subpart D -- Track Structure “Defective Weld”

  5. CFR 49 - Track Safety Standards Subpart F - Inspection § 213.237 Inspection of rail 49 CFR 213.337 “Defective Weld”

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