The Heat Treatment of Ferrous Fasteners: Part II - Post-Processing
The rod may be drawn, or reduced in diameter, which is then classified as wire. The material is then typically cold formed. Cold forming, which was developed for the production of fasteners, consists of forcing metal into dies to form thicker sections and more or less intricate shapes by upsetting and heading.
After the formation of the fasteners, post-processing heat treatments must be performed. This usually is a through-hardening process which consists of an austenitize, quench and temper cycle. The initial hardening of the fasteners is accomplished by heating the components to a level above the critical temperature range (austenitizing), holding at this temperature for a period of time to insure uniform temperature, and quenching (rapidly cooling) to a temperature below the nose of the time-temperature -transformation (TTT or "S") curve for the fastener material being used (Fig. 7). Depending on the specific treatment, the resulting microstructure will contain martensite and possibly minor amounts of retained austenite (Fig. 8).
Quenching is typically accomplished by submersion of the heated fasteners into oil; however, water and polymer quenchants have also been used successfully depending upon the base material of the fasteners being processed. Each of these quench mediums have their advantages and disadvantages, and the proper solution must be chosen to obtain the proper or desired microstructure.
The as-quenched martensite is typically hard and brittle which is not acceptable for fasteners. The fasteners must be tempered to increase toughness while maintaining their high strength. Tempering is accomplished by reheating the fasteners to a temperature well below the eutectoid temperature of the material and holding at this temperature long enough to meet the final desired hardness requirements.
Other post-processing heat treatments include case hardening (gas carburizing, carbonitriding, or a combination of these processes). With these processes, the chemical composition of the material's surface layer (case) is changed by diffusing carbon or a mixture of carbon and nitrogen into the surface of the fastener.
Gas carburizing is a process by which gaseous molecules, carrying carbon atoms, come into contact with the surface of the material being processed. In a reaction with the iron atoms in the fastener alloy, each gas molecule is stripped of one or more of its carbon atoms. The carbon atoms diffuse into the material while the remaining gas molecules are released. The rate of carbon diffusion depends on temperature, carbon concentration gradient, and the diffusivity factor of the material being processed.
FURNACE EQUIPMENT FOR POST-PROCESSINGAs stated in Part I, the successful heat treating of fasteners involves careful control of furnace operating parameters. The temperature, cycle times and atmosphere(s) used to meet the desired metallurgical and mechanical properties are very important. In addition, the type of furnace used adds to this list of parameters. Other considerations such as the total production requirements, amount of acceptable part "nicking," the size of the parts being processed, and the total surface area of parts will help determine the optimum furnace design and whether batch or continuous processing can be utilized.
In the case of batch furnace designs, the selection of furnace type is limited to the requirement of heating and quenching under a protective atmosphere. Tempering, after the hardening and quenching cycle, may be performed in a separate batch tempering furnace to complete the post-processing of the fasteners.
Some of the advantages for batch furnace processing include:
- The overall harden/quench and temper cycles are divided into separate furnaces which allows for production flexibility;
- Atmosphere consumption is typically less than in continuous furnace processing;
- Greater flexibility in changing process parameters for heat treating different products; and
- Batch furnaces accommodate a wide mix of materials and load sizes. This allows for excellent batch control and minimized part mixing.
There are certain disadvantages with batch furnace processing that should also be considered. These include:
- The possibility of non-uniformity of quenching and/or case depths due to a high load density. This results in de-rated load sizes due to high part surface area;
- There is more handling of parts which typically increases manpower requirements.
Developed almost fifty years ago, the batch integral quench (BIQ) design provides for a heating chamber and separate vestibule/quench chamber, allowing for transfer of the workload from the heating chamber into the quenching solution under a protective atmosphere condition. This type of furnace handles the austenitizing and quenching (hardening) cycle of the post processing heat treatment. Typical loading capacity for this style of furnace ranges from 250-385 gross pounds per square foot at 1500ÝF to 215-275 gross pounds per square foot at 1750ÝF depending upon the furnace effective workload zone size. These gross loading values are reduced when processing high surface area parts, such as fasteners, to obtain a proper quenching of the parts being processed. Loads should be limited to a size of 175 to 240 square feet of part surface area for smaller furnaces, and up to 400 to 480 square feet of part surface area for larger furnaces. The high-productivity cloverleaf furnace design (Fig. 11) is similar to the BIQ furnace except that multiple furnace heating chambers are used in conjunction with a common vestibule/quench chamber.
Batch tempering furnaces are designed as companion equipment to batch integral furnaces to provide a means of tempering the hardened and quenched parts. These furnaces are provided with the same physical load sizes and weight capacities as the batch integral furnaces.
Although batch vacuum furnaces can be used for post-processing heat treatments, higher capital equipment costs, higher furnace maintenance, and typically longer cycle times (due to the evacuation time of the furnace chamber) can make these systems cost prohibitive for high volume work.
Continuous Furnace Designs
There are many types of continuous furnaces available for post-processing heat treatment of fasteners. The type of furnace used may be determined by the type of fastener(s) being processed, part size and amount of acceptable "nicking."
Continuous processing provides many advantages over batch processing, some of which include:
- High volume, continuous output of parts, especially for components that have similar process parameters;
- Automatic loading of parts can be provided, reducing required manpower;
- Uniform quenching of parts, since parts are essentially quenched individually;
- Uniform case depths of parts are obtained because a lower density load is being processed on a continuous basis. Furnace atmosphere and temperature remain stable under continuous operation.
Some disadvantages of continuous processing include:
- Dropping, tumbling, or shaking of parts may result in "nicks" that are sometimes not acceptable.
- Changing product mix is more difficult since spacing is required to separate different parts, resulting in reduced production rates.
- Changing furnace line operating parameters (temperatures and atmosphere) is more difficult to accommodate different part materials.
Each furnace design has its own advantages or disadvantages. In many cases, the type of furnace selected depends on the user's preference, past knowledge of the design, or how the furnace line will fit into their overall production line. The most common types of continuous furnaces include:
- Continuous Belt Furnaces
- Shaker (Snap) Hearth Furnaces
- Revolving Retort Furnaces
With continuous belt furnaces, parts are conveyed through the heating and soak zones on a moving conveyor belt. The conveyor belt may be made from woven alloy wire/rod mesh or an assembly of cast alloy "links." At the end of the belt, parts free-fall into a quench tank and are then transported from the quench tank by a conveyor system into a wash section and further into a belt tempering furnace.
Shaker (snap) hearth furnaces utilize a solid hearth that extends inside the furnace. To convey the parts through the furnace, this same hearth is periodically given an oscillating motion (snap) by an externally driven mechanism. The frequency and force of this snap motion can be adjusted to control the total cycle time for the parts being processed. When the parts reach the end of this solid hearth, they free-fall into a quench tank assembly, similar to that previously described, and are conveyed out of the quench medium by a conveyor system.
The revolving retort furnace design utilizes a tube that extends into/through a heated furnace section. This tube has an internal helix that acts as an internal screw conveying mechanism for transporting parts through the furnace. This style of furnace is typically used for small parts where a stirring action may be required, or better suited, such as with case hardening processes.
FINISHINGAfter the austenitizing/quenching/tempering process is completed, almost all fasteners will receive some type of finishing. Some of these finishing processes include a black oxide coating, galvanizing, or electroplating. These finishing processes are performed typically for corrosion resistance and appearance.
ATMOSPHERES FOR PRE- AND POST-PROCESSING HEAT TREATMENTSAtmospheres may be used either as a protective gas to minimize potential contamination of the material surface or for physical surface property enhancements. They can be provided to the heat treating furnaces by gas generating equipment or from bottled gases. In most cases, there is an economic and/or metallurgical advantage for using gas generation equipment. Fig. 12 shows equilibrium curves for different furnace gases used on iron, carbon and steels.
Generated atmospheres are classified in two broad categories, exothermic and endothermic. In both cases, these generators use a mixture of hydrocarbon gas (natural gas) and air to produce an atmosphere of various compositions of nitrogen, hydrogen, carbon monoxide, carbon dioxide, and water (in the form of vapor). This is accomplished through the combustion of the mixture or a reaction of the mixture at higher temperatures. The final atmosphere composition is controlled by varying the air-gas ratio, the reaction temperature, and, in the case of endothermic gas generators, the type of catalyst used.
The DXR exothermic atmosphere gas generator was developed by Surface Combustion in the early 1930s and produces a gas that is most widely used as a protective atmosphere during the annealing process. This type of gas is inexpensive and the raw materials (natural gas and air) for making the atmosphere gas are readily available.
Exothermic atmospheres are produced from the burning of an air/fuel gas mixture in a refractory lined combustion chamber and then cooling the products of combustion in a packed annular cooling chamber. The air/gas ratio of the generator is manually adjusted to produce the required atmosphere composition. The typical composition of a DX generator gas is shown in Fig. 13.
Endothermic Atmosphere Generators
The RX endothermic atmosphere generator was also developed by Surface Combustion in the 1930s and produces a highly reducing gas product for use in heat treating processes such as carburizing, carbonitriding, carbon restoration, and clean hardening of steel. These processes can be performed with the addition of hydrocarbon and ammonia gases to the endothermic atmosphere inside the furnace while operating at the proper process temperatures.
Endothermic atmospheres are produced from the reaction of air and natural gas in a catalyst filled reaction tube that is heated externally. This provides an efficient, low cost, and reliable source of atmosphere gas. Equilibrium charts for RX generated gas are shown in Figs. 14 and 15.