Fasteners come in almost every conceivable shape and size. Fasteners have different thread types (e.g., coarse, fine, extra fine), come in various lengths, with grip or no grip (shank), have different types (e.g., hex, 12 pt, carriage), and different coatings (i.e. passivated, cadmium, dry film lube, hot dip tin, etc.). There are various classes of fit (i.e. class 3) and multiple grades (e.g., grade 2, 5, 8). Fasteners come with left- or right-hand threads, metric or SAE, different number of threads per inch (e.g., 20 or 28 for the same size fastener) and various versions of those (e.g., UNF vs. UNJF).
Fasteners are also made from a variety of different materials (Table 1). Irrespective of their form or material, the heat treatment of fasteners is most often done using mass-production methods, which presents unique heat-treating challenges.
Types of Heat TreatmentsThe most common heat treatments for fasteners involve through or selective hardening and case hardening (carbonitriding and carburizing). Case depths are shallow – typically 0.0015–0.015 inch (0.0038–0.038 mm) specified in ranges of 0.005 inch (0.127 mm). Quench media runs the gamut from brine, water, polymer, oil and salt depending on specification requirements.
In mesh-belt conveyor furnaces (Fig. 3) it is not uncommon to see fasteners loaded between ½–2½ inches (12.7–63.5 mm) deep. Also, whether we like it or not, the reality is that fasteners typically enter the furnace wet and with a certain amount of oil, soap or phosphate residue. If you were to pick up a handful of fasteners off the belt, your hand would be soaking wet and slippery from the amount of oil present. As such we are taxing our furnace atmosphere severely, and atmosphere direction, flow rate and control are critical considerations, as is the performance of the preheat zone. It is not uncommon to see multiple-zone furnaces. For example, a four-zone mesh-belt furnace will normally have circulating fans in the first three zones with zone 1 and 3 running in the same direction and zone 2 running in the opposite direction. Needless to say, the type of fan and fan speed are important considerations.
Induction-heating techniques (Fig. 4) can also be used to heat treat fasteners at high production rates. For example, high-frequency (10–50 kHz) systems can draw back 8620 bolt heads after carburizing to improve toughness at a rate of 1–2½ pieces/second. Similarly, 4140 seat and seat-belt retention bolts – run head down, threads up at 200 kHz – are case hardened to depths up to 0.020 inch and surface hardness of 40–45 HRC to impart both strength and toughness.
Most Common Problems EncounteredFurnace atmosphere problems – often resulting in non-uniformity of case depth or sooting – are the common symptom when the furnace atmosphere is unstable. These problems are due to:
- Part cleanliness (oils, cleaning-compound residue, water, phosphate coatings)
- Air infiltration or products of combustion (air leaks, radiant-tube leaks)
- Zonal separation (temperature, atmosphere)
- Control-thermocouple placement
- Temperature uniformity (side-to-side)
- Temperature profile (down the length of the furnace)
- Atmosphere sampling tools (dew point, CO/CO2/CH4, oxygen probe)
- Sample port location and insertion depth
- Circulating fans (number, type, location, speed)
Important Heat-Treat ConsiderationsFor heat treaters, fastener properties such as strength, toughness and corrosion resistance are important, as is avoiding brittleness and of course cost.
Brittleness is always a concern, and in bolts it is defined as failure at stresses below the strength of the bolt material with little or no evidence of plastic deformation. Typically, fasteners are not brittle below 180 ksi ultimate tensile strength. Nearly all fasteners are considered ductile except some made from certain precipitation-hardening stainless grades (e.g., 15-5 Mo, 17-4 and 17-7).
Toughness is also an important feature of a fastener. It is the opposite of brittleness and gives you an idea of how much abuse a fastener will take without being damaged and eventually weakening – fatigue being a prime consideration. One way to “measure” toughness is by looking at the hardness rating of a fastener. The higher the number (Brinell, Rockwell), the harder the material is and the tougher (more resistant) it is to damage.
Fatigue usually doesn’t play a big part in grade 5 or 8 fasteners since most steels are good for 2 million to 10 million cycles. Almost all fastener fatigue failures are the result of improper (almost always too low) torque. Too low a torque will cause the fastener to pick up more load more often and eventually cycle it to failure. Lubricated threads significantly change the actual preload on the fastener and the risk of over-torque.