Metal testing is a fundamental aspect of welding practice, ensuring that the materials used meet the required standards for strength, durability, and safety. In Kenya’s welding industry, where infrastructure projects and manufacturing depend heavily on metal quality, effective inspection and testing safeguard against failures that could lead to costly repairs or hazardous accidents. This chapter explores the various machines, tools, and equipment used in metal testing, focusing on their operation and maintenance to optimize performance in professional welding environments.

6.1 Metal inspection machines, tools and equipment

Metal inspection is the checking of metal and welded parts to see whether they meet the required size, quality and strength. It is done before welding, to check the material and the joint preparation, during welding, to check the runs, and after welding, to find any defects in the finished part. The work needs the right tools and machines, and they need to be used properly and kept in good condition, because a worn or uncalibrated instrument gives false readings and a good weld may be rejected, or worse, a bad weld may be accepted.

Tools are the simple hand-held or portable items used for measuring, cleaning and examining the surface. Machines are larger instruments that apply a load, send sound waves or radiation into the metal and record how it responds. This topic describes the tools and machines of the metallurgy workshop and the way they are used and looked after, and the tests themselves are covered under the non-destructive and destructive test topics.

6.1.1 Tools

Inspection tools are used to measure dimensions, clean and prepare surfaces, find surface flaws and check the size of welds. The table lists the main tools of this topic.

6.1.1.1 Pneumatic tools

Pneumatic tools are powered by compressed air from a compressor, in place of electricity. In the metal inspection workshop, they are used to prepare the surface of a weld so that defects can be seen. Examples are the air grinder, which has a disc or wire cup brush, the needle scaler, which uses a set of fine needles to knock off slag and scale, and the air chisel for removing weld metal.

Pneumatic tools are light for their power, they do not create a spark from a motor and they do not overheat when they work for a long time. They have to be connected to a clean, dry air supply at the pressure marked on the tool, and a small amount of oil is fed into the air line from a lubricator to protect the moving parts. Hoses must be in good condition and secured, since a hose that is loose can whip about if it breaks away.

Various wire brush attachments for power tools (Source: Wikipedia (Wire brush))

A wire brush is used to clean slag, rust and loose scale off a weld before it is examined, since these hide cracks and pores. Fitted to an air grinder it works faster than a hand brush. The user wears eye protection and gloves, because pieces of wire can fly off.

6.1.1.2 Gauges

A gauge is an instrument used to measure a dimension or to check that it is within the allowed limits. The gauges of a metal inspection workshop include the vernier caliper, the feeler gauge, the weld gauge and the depth gauge.

Digital caliper for precise measurements (Source: Wikimedia Commons)

A vernier caliper measures outside and inside dimensions and depth. The reading is taken from the main scale and the vernier scale together. Example: on a caliper reading to 0.02 mm, the zero of the vernier scale lies just after the 12 mm mark, and the 14th line of the vernier scale lines up with a line on the main scale. The reading is 12 + (14 × 0.02) = 12.28 mm.

A feeler gauge is a set of thin steel blades of different thickness. It is slid into a gap to check the root gap of a joint or the clearance between two parts. A weld gauge has scales or cut-outs to check the leg length, throat and reinforcement of a weld, and to find the angle of a bevel. The gauges are kept in good condition, because a bent or worn gauge reads wrongly.

6.1.1.3 Electromagnets

An electromagnet is a coil of wire wound on an iron core, which becomes a magnet when an electric current flows through it. In metal testing it is used as the electromagnetic yoke for magnetic particle inspection. The yoke has two adjustable legs. When the legs are placed on a test piece of steel and the yoke is switched on, a magnetic field passes through the metal between the legs.

A crack on or just under the surface disturbs the field and makes it leak out of the metal. Fine iron particles sprinkled on the surface are drawn to the leak and form a visible line along the crack. The yoke can be switched on and off, so the part is not left magnetised for long, and it can be carried to the job. It works only on ferromagnetic metals such as steel and cast iron, and not on aluminium, copper or austenitic stainless steel. Before use, the yoke is checked by lifting a steel test weight, to confirm that its field is strong enough. A yoke of the alternating current type commonly has to lift about 4.5 kg at the largest leg spacing used.

6.1.2 Use and care of tools

Tools give correct results only when they are used as intended and kept in good order. The same steps apply to most inspection tools.

1. Check the tool before use for damage, wear and dirt, and do not use one that is bent, cracked or worn.
2. Check the calibration of measuring instruments against a reference standard, such as a gauge block, and record the check.
3. Clean the surface of the part before measuring it, because dirt, rust and slag give false readings.
4. Use the tool for its proper purpose only. A feeler gauge is not a screwdriver and a caliper is not a scriber.
5. Handle precision tools gently and never drop or force them, and avoid squeezing a caliper hard on the work.
6. Clean the tool after use, wipe it with a light film of oil and return it to its case or rack, kept in a dry place.
7. Look at pneumatic tools and electrical leads at the start of the work. Drain water from the air line, lubricate as required and disconnect the supply before changing a disc or a brush.

A calibration record, kept for each instrument, shows the date of the check and the date when the next one is due.

6.1.3 Machines

Inspection machines are larger instruments that test the strength of metal, or find flaws inside it, which cannot be found with hand tools. They are expensive and need trained operators.

6.1.3.1 Universal testing machine

The universal testing machine (UTM) tests the strength of a metal sample by pulling it, or in some tests pressing or bending it, until it breaks. The sample is held in grips at each end, and a motor or hydraulic system draws the grips apart. A load cell measures the force, and an extensometer measures the stretch of a marked length of the sample. A computer plots a graph of load against extension.

From the graph it is possible to find the yield strength, the tensile strength and the elongation of the metal. A machine is selected with a capacity above the highest force expected, and the operator should use the guard around the test area, since samples can fly apart when they break. The same machine can be fitted with other fixtures to carry out compression, bend and shear tests. Hardness and torsion are tested on separate machines.

6.1.3.2 Ultrasonic testing machine

An ultrasonic testing machine finds flaws inside metal using sound waves, at frequencies too high for the ear to hear. A probe containing a crystal sends short pulses of sound into the metal. The sound travels through the metal and is reflected by the back wall and by any flaw in its path, and the probe picks up the echoes. The machine shows the echoes on a screen, and the position of an echo shows how far away the flaw is.

A thin layer of gel is spread between the probe and the metal, to allow the sound to pass. The machine is first set against a calibration block of known thickness. Angle probes are used for welds, so that the sound enters the weld from the side of the plate. The same machine can also measure the thickness of a plate or a pipe from one side only, which makes it useful for checking corrosion.

6.1.3.3 Radiography

Radiography makes a picture of the inside of a weld, in the way that an X-ray picture shows a bone. An X-ray tube or a sealed radioactive source sends penetrating radiation through the part. A film or a digital detector behind the part records how much radiation gets through. Pores, slag and cracks in the weld let more radiation through than the solid metal, so they show up as darker marks on the image.

Radiation can damage the body, so radiography is done only by trained and licensed people, following the rules of the national nuclear regulator. The area is cordoned off and marked with warning signs, and the operators control their exposure by limiting the time, keeping their distance from the source and using shielding. Radiographic images give a permanent record of the weld, which is useful for pressure vessels and pipelines.

6.1.4 Use and care of machines

Machines are costly and any fault in them can produce wrong results or put the operator in danger, so they are used and looked after with great care.

1. Make sure that only trained and authorised people operate the machine, and that they have read the instruction manual.
2. Inspect the machine before use. Check the leads, the guards, the probes and the grips, and make sure that the machine stands on a firm base.
3. Switch on and let the machine warm up, then calibrate it with the reference block or the standard weights before testing.
4. Prepare the sample correctly, and follow the test procedure step by step, using the correct settings.
5. Keep the work area clear, and keep guards and barriers in place when a test is running. Wear the correct protective equipment.
6. After the test, switch off and clean the machine, removing metal dust and couplant, and cover it.
7. Service the machine on the maker's schedule, and record faults, repairs and calibrations in a logbook.
8. Store probes, films and sensitive parts in a clean, dry place and protect them from knocks, dust and heat.

Radiographic equipment is stored in a locked, marked place by the licensed person responsible for it, and the use of the source is recorded each time.

Chapter Summary

This chapter explored the various machines, tools, and equipment used in metal inspection, including pneumatic tools, gauges, and electromagnets, as well as the universal testing, ultrasonic testing, and radiography machines, emphasizing their proper use and maintenance. It examined essential consumables such as films, ferromagnetic materials, and dyes that facilitate testing processes. The chapter detailed key non-destructive metal test parameters like flaws, pinholes, penetration, and undercut, followed by the essential weld preparation procedures involving polishing, grinding, cleaning, and cutting. Various non-destructive testing methods were discussed, including visual inspection, ultrasonic inspection, magnetic particle induction, radiography, dye penetrant, and eddy current testing, alongside their practical applications and procedures. It then covered destructive metal test parameters such as tensile strength, yield strength, hardness, and impact resistance, and described the corresponding destructive testing techniques including tensile, hardness, impact, fatigue, creep, torsion, bend, fracture toughness, and corrosion tests. Finally, the importance of maintenance was highlighted through lubrication, belt adjustments, scheduling, and record keeping to ensure the longevity and accuracy of testing equipment.