Safety in Heat Treatment Furnaces

Heat treatment safety: where do the hazards concentrate?

In Bursa's automotive supplier industry, a large share of gears, shafts, bolts and pressed parts go through a heat treatment such as hardening, tempering or carburising. These processes are carried out either on a furnace line inside the production plant or in subcontract heat treatment workshops. The subject of heat treatment safety is much broader than an ordinary hot surface risk: flammable and toxic gas, molten salt, hundreds of litres of quench oil, heavy baskets and high radiant heat are all present in the same workshop.

Furnace or process	Main hazard	Critical measure
Protective gas (controlled atmosphere) hardening furnace	Explosion with flammable gas, carbon monoxide poisoning	Nitrogen purging, flame curtain, gas detection
Gas nitriding	Ammonia leak	Leak tightness checks, ammonia detector
Salt bath	Steam explosion on contact with water, cyanide and nitrate salts	Dry, preheated parts, separate storage
Quench oil tank	Oil fire, overflow	Monitoring oil level and temperature, lid
Vacuum and induction	Electricity, cooling water leak, nitrogen	Energy isolation, oxygen measurement
Loading and unloading	Falling hot basket, crushing	Lifting equipment checks, danger zone

This table shows why furnaces should not be dealt with in a single line under "hot surfaces" in the risk assessment. Each furnace type should be treated as a separate process.

Protective gas furnaces: explosion and poisoning

A protective atmosphere furnace uses a controlled atmosphere inside the furnace to protect the surface of the part from oxidation or to add carbon. In practice, endothermic gas, nitrogen–methanol mixtures or nitrogen–hydrogen mixtures are common. Endothermic gas contains a significant proportion of carbon monoxide and hydrogen; in other words, it is both toxic and flammable.

A furnace explosion usually happens not during normal operation but at transition points:

If the flammable atmosphere is introduced before the furnace is hot enough, or if air gets in while the atmosphere is inside, a flammable mixture forms.
That is why at start-up the furnace is first purged with nitrogen, and the flammable gas is introduced only after the temperature specified by the manufacturer has been reached. At shutdown the sequence is reversed.
The flame curtain and pilot flames at the door openings burn off the gas that escapes when the door is opened. If the pilot flame has gone out, the door is not opened.
There is a written emergency shutdown sequence for a power cut, the nitrogen supply running out or a control system fault, and operators practise it.

Carbon monoxide is odourless. Fixed gas detectors with audible and visual alarms are installed around the furnace; measurements are taken with a portable detector before maintenance and before entering the furnace. For the ammonia used in gas nitriding, the limit values in Annex 1 of the Regulation on Health and Safety Measures in Working with Chemical Substances are 20 ppm for 8 hours and 50 ppm for 15 minutes. We plan workplace measurements with authorised laboratories.

The area around a furnace using flammable gas is assessed under the Regulation on the Protection of Workers from the Hazards of Explosive Atmospheres and entered in the explosion protection document (Art. 10). For preparing the document, see our article on ATEX and the explosion protection document. The design and inspection principles for furnace safety equipment are set out in the TS EN 746 series of standards and in the furnace manufacturer's operating manual.

Salt baths: contact with water and chemical risks

Salt baths hold the part in molten salt at high temperature in processes such as tempering, austempering and nitrocarburising. The most serious risk is contact with water. When a wet part, a damp basket, a hook holding condensation or water dripping from the ceiling enters molten salt, the water instantly turns to steam and throws molten salt into the surroundings.

That is why the following rules are non-negotiable for salt baths:

Parts and baskets go in dry. Parts coming from washing are dried and, where possible, passed through a preheating furnace.
Salt is topped up dry. Salt bags are stored protected from moisture; damp salt is not thrown into the bath.
There is no water source above the bath. Whether roof leaks, cooling water lines or sprinkler heads could affect the bath is assessed separately in fire planning.
Salts are not mixed. Nitrate and nitrite salts are strong oxidisers; they are kept away from salts containing cyanide or cyanate, oil and organic materials. These salts are stored in separate areas and in separate containers.
Salt containing cyanide never meets acid. Contact with acid releases toxic hydrogen cyanide gas. In Annex 1 of the Chemical Substances Regulation, the 8-hour limit value for sodium cyanide is 1 mg/m³.

Water is not used on a salt bath fire. The extinguishing method is determined from the salt's safety data sheet and written into the emergency plan together with the chemical spill response plan.

Quenching and loading: oil fires and falling baskets

In hardening, the hot part is often cooled rapidly by immersing it in oil. If the part does not enter the oil quickly and completely, the oil may ignite at the surface. To prevent this:

The oil level and temperature are monitored continuously; no quenching is done when the level is low.
Water is prevented from getting into the oil tank; on contact with the hot part, water causes foaming and overflow.
There is a lid on the tank that can be closed in a fire, and an extinguishing system suitable for oil fires.
The speed and sequence of lowering the basket are set out in an instruction.

Baskets are often moved with an overhead crane or a charging car. A falling hot basket means both crushing and burns. Under Annex III of the Regulation on Health and Safety Requirements for the Use of Work Equipment, lifting equipment and pressurised gas tanks are subject to periodic inspection once a year unless the standards set a shorter interval. The suitability of chains and slings used at high temperature is checked against the manufacturer's values. For details, see our article on overhead crane safety.

High temperature: burns and heat stress

When the furnace door is opened, the operator is exposed to intense radiant heat. The order of measures again starts with engineering:

Distance and automation: Charging cars, long-handled tools and automatic doors shorten the time the operator spends at the door opening.
Shielding: Heat curtains and reflective panels reduce radiant heat.
Personal protective equipment: Aluminised apron and sleeves, heat-resistant gloves, a face shield with an infrared filter and leather footwear.
Work organisation: Rest breaks in hot periods, a shaded and cool rest area, and drinking water available at all times.

Our article on industrial heat stress and WBGT provides guidance on assessing the heat load. The workplace physician assesses the fitness of workers with cardiovascular disease or heat sensitivity to work at the furnace. Share our article on first aid for burns with the first aiders in the furnace area for first response to burns.

Maintenance and entering the furnace

Entering a furnace for maintenance is work in a confined space. A furnace purged with nitrogen has no oxygen; it is impossible to tell this by looking. Before entry:

All energy and gas supplies are disconnected, locked and tagged; blanking plates or double block and bleed are applied to gas lines. For the method, see our article on lockout-tagout.
The furnace is ventilated and oxygen, carbon monoxide and flammable gas are measured; measurement continues throughout the work. Our article on confined space gas testing provides guidance on the measurement sequence.
There is an attendant outside, and the rescue method is determined in advance.
For dust generated when removing refractory and insulation, the material's safety data sheet is read and suitable respiratory protection is selected.

Heat treatment furnaces do not appear as a separate heading in the tables in Annex III of the Work Equipment Regulation. The Regulation requires that, where no interval is specified in the table or in standards, periodic inspection be carried out at the interval set by the manufacturer, or failing that, according to the result of the risk assessment. The electrical installation, pressurised gas tanks and lifting equipment connected to the furnace are subject to the intervals in their own tables. You can track these dates with our OHS calendar tool.

Under our risk assessment and OHS specialist services, we assess heat treatment lines separately by furnace type and write start-up and emergency shutdown instructions on site together with the operators. For other risks in the sector, see our metal and machinery page.

Frequently Asked Questions

Can water be used on a salt bath fire?

No. Water in contact with molten salt instantly turns to steam and throws the salt into the surroundings. The extinguishing method is determined from the salt's safety data sheet and written into the emergency plan.

Why is endothermic gas dangerous?

Endothermic gas contains carbon monoxide and hydrogen. Carbon monoxide is odourless and toxic; hydrogen and carbon monoxide form a flammable mixture with air. That is why gas detection, nitrogen purging and flame curtains are used together.

Is a heat treatment furnace subject to periodic inspection?

The furnace is not listed separately in the Annex III tables. Periodic inspection is carried out at the interval set by the manufacturer, or failing that, according to the risk assessment. The electrical installation, pressure tanks and lifting equipment connected to the furnace are inspected once a year.

What training should furnace operators receive?

In addition to basic OHS training, the start-up and shutdown instruction specific to the furnace type, what to do in a gas alarm, salt bath rules, first aid for burns and the safe use of lifting equipment should be taught on site and in practice.