SOLAS Metal Fabrication key terms

A metal made by melting two or more elements together, at least one of them a metal, so the mixture gains properties such as strength or corrosion resistance that the pure metals lack.

Any metal whose main ingredient is iron. Mild steel, cast iron and stainless steel are examples. Ferrous metals are magnetic and will rust unless protected or alloyed against corrosion.

A metal that contains little or no iron, such as aluminium, copper, brass or zinc. These metals resist rust, are often lighter, and are chosen where weight or corrosion resistance matters.

A low-carbon steel with roughly 0.1 to 0.3 percent carbon, widely used in fabrication because it is cheap, easy to weld and form, but it corrodes quickly without a protective coating.

A steel alloyed with at least about 10.5 percent chromium, which forms a thin self-repairing oxide layer on the surface. This layer makes the metal highly resistant to rust and staining.

A light, soft, silvery non-ferrous metal about one third the density of steel. It forms a natural oxide skin, conducts heat well, and is often alloyed for strength in fabrication work.

An iron-carbon alloy with high carbon content, usually over 2 percent. It is brittle and hard, flows well when molten for casting, and is used for machine bases, pipes and heavy components.

The percentage of carbon in steel or cast iron. Higher carbon makes metal harder and stronger but less ductile and harder to weld, so the figure guides which processes and electrodes are suitable.

The ability of a metal to be stretched or drawn into a thinner shape without breaking. A ductile metal bends rather than snaps, which matters when choosing material for forming and for structures under load.

The ability of a metal to be hammered or rolled into thin sheets without cracking. Malleable metals deform under compressive force, which is why they suit pressing, rolling and shaping operations.

The maximum pulling stress a material can withstand before it breaks or necks down. It is measured in newtons per square millimetre or megapascals and is quoted on material test certificates.

The stress at which a metal stops behaving elastically and starts to deform permanently. Once past this point, the material will not spring back to its original shape when the load is removed.

The stress that produces a small permanent strain, commonly 0.2 percent, used instead of a yield point for metals such as aluminium that show no clear yield. It is a standard design value.

Controlled heating and cooling of a metal to change its internal structure and so alter properties such as hardness, toughness or machinability. Processes include annealing, normalising, hardening and tempering.

A heat treatment that heats metal to a suitable temperature and cools it slowly, usually in a furnace. It softens the metal, relieves internal stresses and restores ductility after working.

A heat treatment that heats steel above its critical temperature and quenches it rapidly in water, oil or air. The result is a very hard but brittle structure, usually tempered afterwards.

A heat treatment carried out after hardening that reheats the steel to a lower temperature and cools it. It reduces brittleness and relieves stress while keeping an acceptable level of hardness.

Rapid cooling of hot metal by plunging it into water, oil, polymer or forced air. The cooling rate controls how hard the metal becomes and can introduce distortion or cracking if uncontrolled.

The increase in strength and hardness, and loss of ductility, that occurs when a metal is deformed cold by rolling, bending or hammering. Annealing is used to reverse the effect.

How readily a metal can be welded to give a sound joint without cracking or unacceptable loss of properties. It depends on composition, thickness and the welding process chosen.

Coating steel with zinc, usually by hot dipping, to protect it from rust. The zinc corrodes in preference to the steel and so gives sacrificial protection, even where the coating is scratched.

An electrolytic process that thickens the natural oxide layer on aluminium, improving corrosion resistance and allowing dyeing. The coating is integral to the metal rather than applied on top.

A fixed reference point, line or surface from which measurements and dimensions are taken. Using a common datum prevents small errors accumulating when marking out and checking a fabricated part.

Transferring dimensions and shapes from a drawing onto metal using rules, scribers, dividers and squares. Accurate marking out is the basis for cutting and forming parts to the required size.

A sharp-pointed hardened steel tool used to scratch fine lines on metal during marking out. It leaves a permanent mark but removes a little surface material, so it is not used on finished faces.

A pointed steel punch struck with a hammer to make a small conical indentation. The mark guides a drill so it starts in the right place and does not wander across the surface.

The permissible amount by which a dimension may vary from the stated size while still being acceptable. Tolerances are shown on drawings and control how precisely a part must be made.

The intentional small gap between two mating parts, such as a bolt and its hole or a punch and die. Correct clearance allows assembly, movement or cutting without binding or excessive play.

Cutting a round hole in metal with a rotating fluted tool called a drill. Correct speed, feed, coolant and clamping give a clean, straight hole of the intended diameter.

Enlarging a drilled hole slightly with a multi-edged reamer to bring it to an accurate diameter and a smooth finish. It is used where a close fit for a pin or shaft is required.

Cutting an internal thread into a drilled hole using a tap held in a tap wrench. The hole is drilled to a specified tapping size slightly smaller than the thread so the tap can cut.

A tool with an internal thread that cuts an external thread onto a rod or bolt when rotated. The term also refers to the shaped tool used in presswork to cut or form sheet metal.

Cutting sheet or plate metal between two blades that slide past each other. The metal fails along a line where the blades meet, producing a cut edge with slight distortion.

A machine with a long straight blade that descends to shear sheet metal along a straight line. Guards and hold-downs keep hands clear and stop the work lifting during the cut.

A hardened tool that is driven through or into metal to cut holes or form shapes, usually in a press. It works with a matching die to shear the material cleanly.

Unwanted change in the shape or straightness of a part, caused by uneven heating, welding shrinkage or machining stress. It is controlled by balanced sequences, clamping and stress relief.

A joint made by melting or otherwise fusing metal at the contact faces, often with a filler material. The finished joint must be strong, sound and free of cracks or porosity.

The melting together of the parent metal, and usually a filler rod or electrode, to form one continuous joint. Enough heat must reach the joint faces for fusion to occur throughout.

The depth to which the weld metal fuses into the parent metal below the surface. Adequate penetration is essential for joint strength and is checked by inspection or test pieces.

The band of parent metal beside a weld that is changed by the heat of welding but not melted. Its properties can differ from the rest, sometimes becoming harder or more brittle.

Cleaning and treating a metal surface before coating, painting or welding, by removing rust, scale, oil and dirt. Good preparation is essential for adhesion and for sound welds.