The metal parts of diagonal cutting pliers are made of steel and the handle coverings are made from plastic.
Steel is an alloy (mixture) of mainly iron and carbon with small quantities of other elements.
Increasing the amount of carbon makes the steel harder but brittle (more likely to break). Adding manganese to the alloy makes it less brittle and improves its ability to be shaped with a hammer when hot.
Different alloys are more suitable than others for making diagonal cutting pliers. Beyond inspecting the tool for blade sharpness, the quality of the steel and the blade strength is sometimes hard to judge except through the price of the tool. The more expensive or longer the guarantee, the better quality the steel is likely to be.
Some tools are made from ‘chrome vanadium’ steel. Chromium and vanadium are metal elements that are added to steel to increase the metal’s strength. Chromium also increases the steel’s resistance to rusting.
The overall chromium content of steel is usually between 12% and 20% while the overall content of vanadium should be between 0.2% and 1%.
Vanadium is expensive so cheaper chrome vanadium steel products often have a very low vanadium content, as low as 0.1%. Inspecting the tool to ensure the cutting blades are clean, not chipped and don’t have any metal burrs while taking into account the relative price of the tool should give you an idea of the quality of the alloy.
Polypropylene is a common type of plastic used for the handles of diagonal cutting pliers. It has a high melting point and can withstand being dropped and knocked about so will survive on a work site.
Adding an elastic material to polypropylene or another thermoplastic (plastic that can be melted and set in a mould) can make a rubbery plastic which is useful for overmoulding (see How are diagonal cutting pliers manufactured) onto the handles for extra grip and comfort.
Vinyl is used for simpler handles where just a layer of soft plastic is required for grip. It is made from ethylene (found in crude oil) and chlorine (found in salt) and is a cheaper alternative to other plastics.
All these plastics can be coloured to produce brightly coloured handles that are easier to find on the work site.