Today’s tight quality requirements together with economic constraints have led to the advancement of fastener-furnace technology, with belt-type furnaces resulting as the dominant design. This article generally describes the process of hardening and tempering and the plant technology commonly employed in the modern fastener industry. Emphasis is put on precise process monitoring of the bulk flow with regard to the uniform final properties like tensile strength and toughness of each fastener.

Fig. 1. Partial view of a hardening and tempering line with a nominal throughput of 2,000 kg/h


Hardening and tempering is arguably the most important heat treatment in the fastener industry. For automotive safety parts, this important production step is almost always employed. The basis for the heat treatment of fasteners is the standard EN ISO 898-1 that prescribes a homogenous hardened and tempered steel structure. Carbon contents of 0.25–0.35% result in the best values for combined tensile strength and toughness both for non- and low-alloyed steels. A tensile strength ranging from 800–1,300 N/mm² may also be achieved under certain conditions without final heat treatment if cold forging and aging or micro alloying are employed, but these have had limited application up to now.

Fig. 2. Cast link-belt furnace for a production rate of 3,000 kg/h with opened maintenance front door

Furnace Technology

Requirements for fastener characteristics demand a specially designed furnace, reliable plant technology, field experience and knowledge of physical and metallurgical process parameters and their influence on heat-treatment results. This is especially true for mass-produced, high-grade fasteners with diameters ranging from smaller than 2 mm to larger than 50 mm. Generally, it’s expected that every defect-free piece will have defined properties with small dispersions of hardness, tensile strength, toughness and fatigue strength among others.

The furnace-plant design targets may be summed up as:
  • Highest process capability within narrow tolerance bands and reliability of plant parameters
  • Plant availability and productivity
  • Lowest life-cycle costs regarding operating (media consumption) and service life (preventive maintenance)
  • Minimum environmental impact
To achieve these objectives, fasteners are chiefly heat treated in continuous furnace lines tailored to customers’ needs and possibly geared toward an almost identical ambient-temperature gas/fluid impact on the single pieces in the bulk flow. The characteristic statistical values describing the degree to which the required properties are satisfied are Cmk and Cpk, for machine and process respectively, with the factor “k” further indicating the position of the scatter band between the specified tolerance limits. The k values can often be centered by adapting the tempering temperature, for instance. A commonly accepted value for Cmk or Cpk is >1.67. The tolerance bands for the tensile strength prescribed for the various grades in EN ISO 898-1 are often further reduced to 50 MPa, especially by the automotive industry. Precisely observing the theoretic temperature curves of hardening and tempering becomes secondary to the homogenous distribution of the properties of each fastener, and the continuous monitoring of parameters like temperature, recirculation flow and protective-gas composition is crucial to production.

Due to their economic disadvantage with large production runs, batch-type furnaces are used for special applications like positioning long and slim fasteners that are subject to bending or for improved flexibility with small charges.

A complete belt-furnace-line installation for hardening and tempering is shown in Fig. 1. It typically comprises (in sequence) a container’s unloading station, a prewashing and dephosphating machine, the hardening furnace, an oil quench, a postwasher, the (bright) tempering furnace with subsequent soluble oil cooling and a container refilling station.

Continuously operating belt-type furnace lines for fasteners can be divided into either cast link-belt furnaces or wire mesh-belt continuous furnace lines. Cast link-belt-type furnaces are employed for the hardening and tempering of more rugged fasteners from M5 to M48 with throughputs from 500 to 3,000 kg/h (Fig. 2). Additional characteristics of these furnaces are that the rugged belt can be charged with approximately twice as much load as a wire mesh belt, and the furnace entrance may be closed by a protective gas lock. Both characteristics considerably reduce the heat and protective gas consumption. The furnace is usually gas heated with recuperative ceramic or cast-steel radiation tubes, and it is economically best suited for large-scale production with large heat-treatment lots.

Wire mesh-belt-type continuous furnace lines are ideal for fasteners from M2 to M24 with smaller throughputs from approximately 10 to 1,000 kg/h (Fig. 3). Further characteristics of these furnaces:
  • Appropriate for smaller fasteners
  • The belt charging can be monitored easily from the outside
  • The belt drive is made as a moving honeycomb hearth plate or on support rollers
  • Electric heating for smaller throughputs
The continuous washing machines are available as a drum and conveyor version for alkaline cleaning of the fasteners before and after hardening. The prewashing also comprises a dephosphating stage required for grade 12.9 by EN ISO 898-1 since the phosphate created by the wire coating may result in dangerous surface decarburization. The postwasher, apart from cleaning off the quench oil, should also guarantee that the fasteners are properly dried before entering the tempering furnace.

Fig. 3. Section of a wire mesh-belt hardening furnace with integrated oil quench for a throughput of 500 kg/h (Courtesy of SAFED)

Hardening

Hardening temperatures for fasteners normally range from 880–900°C (1600–1650°F). The purpose of the furnace is to homogenously austenitize and dissolve the ferrous and alloy carbides in the steel because carbides or lower carbon content in the austenitic phase deteriorate the hardness of the fasteners upon subsequent quenching. The process is carried out under neutral endothermic protective atmosphere enriched with either natural gas or propane as correctives. Alternatively, nitrogen with methanol cracked inside the furnace might be used. The control of the carbon potential to adapt it to the fastener steels’ carbon content is done for both processes continuously with sampling tools like Lambda or oxygen probes.

The time for austenitizing and carbide dissolving with cold-formed, low-alloy fasteners varies between 10 and 15 minutes, with previous good annealing for easier cold forming prolonging this time. Add to this the heating up and soaking times with bigger diameters and higher bulk heights on the belt requiring more time. This implies that the throughput °° (bulk height)/(furnace residence time) depends on the fastener dimension, bulk density and temperature distribution in the furnace. An example of a measured temperature curve of fasteners and derived resident times in a hardening furnace is shown in Fig. 4.

The drop chute for the transfer of the fasteners into the quench bath is heated to prevent the austenitized fasteners from cooling prematurely and remaining soft. The quenchant curtain hinders rising vapor from entering the furnace. Above it, on the inside of the drop shaft, a gas-suction device further shields the protective atmosphere.

The martensite hardening of high-grade fasteners from boron or higher-alloyed steel is done in mineral quench oil to achieve the critical-cooling speed necessary for the austenitic transformation and to avoid bending or even cracking. The falling depth depends primarily on the fasteners’ diameter, then on its alloy content. The falling speed in the quenchant varies from 1–2 m/s – nearly independent of the size.

The discharge is almost always accomplished using a belt conveyor with bolted-on flights, with smaller fasteners also being extracted by means of a bucket-chain or magnetic conveyor.

Since most fasteners, due to heat-transfer physics, need a rather long time to cool down, they hit the belt being still in their soft, red-incandescent, austenitic state. As an example, the time for a screw M16 of 20MnB4 steel to cool down to 500°C (930°F) is approximately 10 seconds. This means that the minimum quenching speed according to the steel quality must be fulfilled by sufficient delivery of quenchant into the impact area. If the belt is running too slowly with the aim to extract the fasteners rather cold to avoid oil smoke, the fasteners might amass on the flights and worsen the quenching speed for each fastener due to the lack of circulation flow. The quenching technique supposes that the fasteners’ alloy content is high enough to reach the critical quenching speed in the fasteners’ total section. For big bolts up to M48, alloys like 34CrNiMo6 have to be used to harden the core. The risk with big bolts is cracking due to the high inner tensions by heat transfer during quenching. The known hardness of the fasteners based on their carbon percentage with 99.9% martensite can easily be used to confirm the completeness of austenite transformation by taking samples during production.

Fig. 4. Thermocouple drag test of temperature uniformity in a hardening furnace with 1,000 kg/h

Tempering

Tempering controls the final properties of the fasteners. The tempering temperature and time of approximately 60–120 minutes are the parameters, which determine final tensile strength and toughness. As a rule of thumb, the higher the temperature and the longer the dwelling time of the fasteners in the tempering furnace, the lower the tensile strength and the higher the toughness values. The disintegration speed of the martensite is also an exponential function of the still untransformed martensite so that the times both for heating up and dwelling at temperature are summed up.

To get results within narrow tolerance bands it is not sufficient to rely on set recipes for carbon/alloy content and fastener grade. The metallurgical analysis (18 elements) for each heat-treatment lot has to be performed for setting and fine-tuning the furnace parameters. Another important quality-determining factor is the uniform distribution of the fasteners on the belt. If they pile up, heat-up and soak times can vary so that final mechanical properties scatter significantly.

The tempering furnaces are also wire mesh-belt or cast link-belt conveyor types, matched to the throughput of the hardening furnace. The throughput is again a function of the bulk density. Since the heat transfer in the typical tempering range from 350–570°C (660–1060°F) depends mainly on the convection imparted by the axial cyclones or fans, a lower bulk density (i.e. slim fasteners) allows for higher bulk heights with final properties remaining constant. To achieve the powerful atmosphere recirculation, the furnace is divided into multiple zones and precisely controlled.

Alternatively to the air or nitrogen atmosphere, the so-called bright-tempering process with protective gas is utilized. The aim is to reduce the risk of hydrogen embrittlement. Since the modern zinc lamellar surface coatings require a bright surface (no tempering colors allowed), the pickling is normally done to reduce the oxide layer so that a surface preparation may be reduced or even eliminated.

After tempering, the fasteners are quenched in a soluble oil bath 1 meter deep. In most cases, the desired surface property for fasteners is still blackened/oiled. The pores in the Fe3O4 oxide layer of 1-3µm thickness are filled with oil residues. To create this blackening, a tempering temperature of 400°C (750°F) minimum is required. The resulting surface protects sufficiently against corrosion during storage of the fasteners.

Common Problems Arising with the Heat Treatment of Fasteners

The bulk production of fasteners entails specific problems that must be addressed and minimized during design and operation of the continuous furnace plants. Most important are:
  • Damages on threads due to handling of the bulk flow, especially in the container-loading station and prewashing machine when fasteners are still soft. Fine threading might be done only after the heat treatment.
  • Mixing of different heat-treatment lots on the belts.
  • Bending of fasteners with a length/diameter ratio of >12. These fasteners must be straightened afterwards or heat treated on a rack in batch-type furnaces.
  • Contamination by alkaline deposits from the high-temperature furnace and its influence on the heat-treated fasteners. This can be reduced by a double-rinsing stage after dephosphating in the prewashing machine and employing a triple-water cascade with fresh water directly sprayed on the fasteners in the final washing stage.


Conclusion

The challenge in the heat treatment of fasteners lies chiefly in the ability to mass produce while requiring that each piece exits the process with the same values of final tensile strength and toughness. Today’s furnace developments are aimed at reducing the statistical tolerance bands of these properties. IH

For more information: Contact Udo Brenner, president, Aichelin Heat Treatment Systems Inc., 44160 Plymouth Oaks Blvd., Plymouth, MI 48170; tel: 734-459-9850; fax: 734-459-9851; e-mail: U.Brenner@aichelinusa.com; web: www.aichelinusa.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: fastener, cast link belt, wire mesh belt, recuperative ceramic tube, radiation tube, endothermic atmosphere, tensile strength, toughness, martensite