Three different heat treatments
A fastener can see heat treatment at three points in its life, each with a different purpose:
- Annealing of the wire before forming, to make it soft enough to cold head.
- Hardening and tempering of the finished part, to give it its property class.
- Baking after electroplating, to drive out hydrogen (see coatings).
Quenching and tempering, step by step
- Austenitising. Parts travel on a mesh belt through a furnace with a controlled, carbon-balanced atmosphere. At hardening temperature (typically around 850–900 °C for these steels) the structure changes to austenite. The atmosphere stops the surface from losing carbon or picking up extra carbon.
- Quenching. The hot parts drop into an oil bath and cool rapidly. The structure turns into martensite: very hard, very strong, but brittle.
- Tempering. Parts are reheated to a lower temperature and held. Some hardness is traded for toughness and ductility, so the bolt stretches before it breaks instead of snapping.
Furnace lines run 24 hours a day, with temperatures, times and atmosphere recorded for every batch. Automotive suppliers audit this against CQI-9.
The rules in ISO 898-1
- Classes 8.8, 9.8, 10.9 and 12.9 must be quenched and tempered.
- The minimum tempering temperature is 425 °C for these classes (with one exception for certain 12.9 steels). A high tempering temperature makes sure the bolt is properly toughened rather than left hard and brittle.
- The steel must harden through: about 90% martensite in the core of the threaded section after quenching. This is why larger bolts need alloy or boron steels with more hardenability.
- The surface must not lose carbon (decarburisation) or gain it (carburisation) beyond set limits. Both are checked in the test lab.
- Classes 4.6 to 6.8 do not need hardening. Classes such as 4.8 and 5.8 get their strength from cold work during forming.
Case hardening: self-tapping and thread-forming screws
Self-tapping, self-drilling and thread-forming screws must cut or form their own thread in sheet metal, so the surface needs to be very hard, while the core stays tough enough not to snap. These screws are made from low-carbon steel and case hardened: carbon (and sometimes nitrogen) is diffused into a thin surface layer, then the part is quenched and tempered. This is very different from an 8.8 bolt, which is hardened evenly through its whole section.
Nuts and washers
Nuts of class 8 in common sizes are often made from steel that is strong enough as formed. Class 10 and 12 nuts, and larger class 8 nuts, are quenched and tempered like bolts. Hardened washers for structural bolting are also quenched and tempered to a set hardness so they do not bed in under load.
Why stainless steel is not hardened
A2 and A4 stainless steels are austenitic. They do not form martensite when quenched, so they cannot be hardened by heat treatment. Their strength comes from cold working during forming. Martensitic stainless grades (the C groups in ISO 3506) can be hardened, but are less corrosion resistant.
Heat treatment in Indian practice
- Continuous mesh-belt furnaces with endothermic or nitrogen-methanol atmosphere and oil quench are used by the larger bolt makers, especially those supplying the automotive industry, many of whom are audited to CQI-9.
- Sealed-quench batch furnaces are common with job-work heat treaters who serve smaller fastener units. Done properly they give good results; the risk is mixed loads and poor records.
- Pit furnaces are used for long studs, anchor bolts and large foundation bolts that will not fit on a belt.
- EN19 and EN24 studs are usually hardened and tempered by the stud maker or a heat treater to the hardness or strength on the drawing, and checked by hardness test.
Typical problems we see in the market are surface decarburisation (soft threads) from uncontrolled furnaces, and over-hard bolts tempered too low. Both are found by hardness testing across the section and by a microstructure check, so ask for hardness results on the test certificate for 10.9 and 12.9 bolts.
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.