The Heat Is On

Mannings Thermal and Environmental Engineers of Southport, England are one of the worlds leading specialists in heat treatment of materials. They manufacture, sell and hire a wide range of equipment for numerous heat treatment processes and provide after-sales train courses and advice. They describe some of the principles involved and explain the latest heat treatment technology. The many advances in thermal treatments have meant that any size or shape of object can be handled. The technology is continually being improved and applied to new materials and components. Mannings operate a policy of ongoing research and development and are members of the Norweb Business Partnership Scheme, EMTEC.

HISTORY OF HEAT TREATMENT

Heat treatment of materials is a fundamental metallurgical process, which dates back thousands of years. The earliest evidence of heat treatment was on a dagger, which had been forged and tempered in Egypt around 1350 BC. The Blacksmiths art had its foundations in the Iron Age and for centuries blacksmiths have used heat in the manufacture of armour, tools and many other metal objects. The equipment and techniques used in modern heat treatment processes have advanced rapidly and are applied to plant and components for oil refineries, chemical processes, shipbuilding, power stations, aircraft manufacture and many more.

WHY HEAT TREAT?

Materials can be subjected to heat treatment to relieve internal stresses, reduce brittleness and to improve machinability. Other properties of materials can also be altered such as hardness, strength, toughness and wear resistance to suit particular applications. In the case of plain carbon steels, for example, heat treatment processes can change the structure and physical properties. Heating and slowly cooling these materials will relieve internal strains and induce 'softness'. On the other hand 'hardness' is achieved by heating and cooling rapidly.

WHAT HAPPENS DURING HEAT TREATMENT

Indeed the basic principle involved in heat treatment is simply the process of heating and cooling. However the underlying changes induced in materials under different conditions are much more complex. When steel is heated, it's temperature rises at a constant rate until it reaches approximately 7200 Centigrade, which is known as the 'lower critical point'. At this point the temperature of the steel remains constant - even though heat continues to be applied. After a short time the temperature will begin to rise again. In plain carbon steels a second and third critical point can also be observed at higher temperatures.

A remarkable phenomenon occurs when steel is allowed to cool at a uniform rate. Under these circumstances the metal actually evolves heat at certain points even though the temperature remains steady. This effect is known as recalescence and can be seen in the dark. The critical points on cooling take place at lower temperatures than the corresponding points during heating. The main heat treatment processes produce material for specific purposes and which we take for granted.

COMMON HEAT TREATMENT PROCESSES

ANNEALING - This was the first form of heat treatment which was applied to ferrous metals. It is a process of controlled heating and slow cooling which produces a soft machinable metal. This procedure also removes gasses, relieves stresses and can enhance mechanical properties.

NORMALISING - Is an accelerated annealing process since the cooling period is much faster. This results in increased strength and impact resistance but is achieved at the expense of ductility.

QUENCH - Is the process of cooling heated metal to room temperature very HARDENING quickly. Metal is raised above its upper critical temperature and cooled by immersing it in liquid or gas. This treatment produces a hard and brittle material.

TEMPERING - This is the process of heating and slowly cooling a quench hardened steel to relieve internal stresses. The temperatures are varied to produce different properties depending on the application of the steel. Almost all tool steels are treated in this way.

APPLICATIONS FOR HEAT TREATMENT

Some of the most common forms of heat treatments are those which are applied to steel vessels and pipe work in the form of PREHEAT, CONCURRENT HEAT and POSTWELD heat treatments.

PREHEATING - This is the process of heating a weld zone before welding commences and is an effective method of preventing cracks appearing in the weld or parent metal by eliminating undesirable stresses and driving off water.

CONCURRENT HEAT - This is the process of applying heat while welding is taking place. This has the beneficial effect of preventing heat loss and slowing down cooling as well as removing hydrogen from the weld and reducing residual stress.

POSTWELD HEAT TREATMENT - This is of considerable importance in the fabrication of many steel vessels and piping systems. This is designed to reduce internal stresses caused by welding and soften any tempered areas of the weld or of the metal in the vicinity of the weld. The method of postweld heat treatment is to heat to the required temperature, hold at this temperature for a defined time period and to cool at a controlled rate. The heat treatment cycle is governed by the particular code of practice being followed e.g. ASME, BS or DIN.

FURNACE HEAT TREATMENT

Heat treatments on welded parts can be carried out in a workshop or on site and there is no limit to the size or shape of piece, which can be treated. The parts can be placed in a heat treatment furnace or can be treated by localised heat on the inside or outside.

In a furnace, heat treatment is carried out under controlled conditions. Many years ago these furnaces were lined with firebrick and were very heavy and expensive to run. Modern furnaces are lined with low thermal mass insulation and are usually heated by gas, oil or electricity. By the use of fully automatic furnace control systems it is possible to achieve very accurate temperature control. Fuel savings of up to 47% are experienced with modern furnaces compared to the traditional firebrick designs.

Furnaces are manufactured in a variety of configurations to suit the particular situations such as Roller Hearth, Roller Cover, Top Hat, Bogie Hearth, Multiple Hearth and many more. In addition special furnaces are continually being developed to suit modern heat treatment applications, for example those providing controlled atmospheres. Standard and special furnaces are manufactured and shipped as complete units or sold as Self Build units, which are more economical since only the technical expertise, and special components need be provided.

LOCALISED HEATING FOR PIPE WELDS

Another common and simple method of heat treatment of a pipe or vessel joint is to use electric flexible ceramic pad heaters. These pads consist of multi-strand nickel chrome element wire which is electrically insulated by the use of interlocking sintered alumna beads to produce a flexible mat with high heat transfer capacity. The mats are simply wrapped around the part to be heat treated and covered with insulation to limit heat loss. Safe voltage heaters are rated at 30, 40, 60, or 80 volts and mains voltage heaters are also available. Electrical joints are made by the use of special 'camlock' connectors.

Electric heaters are manufactured in a variety of forms such as rope heaters for use in awkward situations, braided heaters for non-uniform shapes, shoelace heaters for small components and special purpose insulated pre heaters. For speed and efficiency a range of Pipe Butt Clamp On Heaters are available. These are pre-formed heater assemblies, which are produced in a variety of sizes to suit specific pipe diameters and are indispensable for a pipe work production workshop where a fast throughput is required.

RADIANT HEAT FOR VESSEL SEAMS

For longitudinal seams on large vessels or for circumferential seams on vessels, which are rotating during automatic welding operations, electric, or gas infra red heaters can be used to provide a means of preheat. These Electric Infra Red Heaters and Radiant Preheat gas burners can be used singly or in multiples, positioned vertically, horizontally or circumferentially.

SITE HEAT TREATMENT OF LARGE VESSELS

When large vessels require Postweld heat treatment, one solution is to turn the vessel itself into a furnace. By using an electric, gas power source, the internal heating technique can be employed. In the case of a gas-fired process, heating is achieved by the use of portable, high velocity combustion units and the burners are fired into a vessel, which is externally insulated. These burners have an exceptionally high turndown capability and can accurately maintain temperature set points. This method is frequently used for on-site treatment of large pressure vessels such as LPG storage spheres.

CONTROL SYSTEMS

A heat treatment cycle can be controlled manually or it can be carried out under fully automatic control by the use of modern control systems. To control electrically powered equipment, self-contained Heat Treatment Centres have been designed to provide the power source as well as programmed temperature control and recording. As with all equipment used for heat treatment, the range of control units need to be of a robust construction to cope with the types of environment encountered on construction sites. Stainless steel heat treatment units are available for use in countries, which experience high humidity and are even used on offshore oil platforms for sub-sea welding operations.