FORMING PROCESSES Production of stress-free quench and temper bars in high volume continuous furnaces Historically, the greatest challenge in producing pre-hardened quench and temper bars has been the difficulty in achieving the mechanical and metallurgical properties while maintaining acceptable straightness. This is particularly difficult with small diameter (20-70mm) bars where creep deflection during austenitising can be significant. The net result is that many processes require costly post quench and temper (Q&T) straightening that results in uneven and sometimes large residual stresses in the bars. Often these residual stresses lead to machining irregularities and movement during machining at the end-user s plant, both undesirable outcomes. This paper will discuss the production of quench and temper stress-free bars that do not require post Q&T straightening. Authors: Michael K Klauck and Gregory R Stanley Can-Eng Furnaces International Ltd The growth in pre-hardened alloy steel bars in recent years has been significant. The driving force behind this has been the ability of machining centres to cut at ultra-high speeds, combined with improved coatings for carbide cutting tools. This makes the machining of a prehardened 32 HRc (297 BHN) steel bar a cost-effective task for conversion into a drive shaft, axle or a similar power transmission component. For the machine building industries there are literally hundreds of applications that make use of pre-hardened steel bars, the majority of which are shipped from distributor warehouses around the globe. Pre-hardened steel bars offer the end-user many advantages in that the component can be machined to size and no subsequent heat treatment is required. This avoids having to leave additional machining tolerances for heat treatment distortion, and is particularly useful in long product heat treatment where the length to diameter ratio is such that warpage is a real issue. This places the onus for straightness back on the mill source to supply a bar that is generally of the order of 2mm per 1m length of deviation, or better. In the past this was achieved by running the steel bars through a large rotary or gag straightener to correct warpage during the heat treatment cycle. While this produced bars of commercially acceptable straightness the straightening operation often left the bars with localised uneven stresses that were usually detected in the form of movement (distortion) during end-user machining operations. To that end, manufacturers of industrial furnaces have sought methods to produce stress-free prehardened steel bars, such that uneven residual stresses are r Figs 1a and 1b Car furnace not present, and no subsequent straightening is required after heat treatment. SHIFTS IN HEAT TREATING TECHNOLOGY Conventional heat treatment of long products (bars) consisted of quenching and tempering (Q&T) in either batch car bottom (bogey hearth; see Figures 1a and 1b) or roller hearth furnaces. Bars produced from both systems a 155
r Fig 2 Immersion quench in PAG r Fig 3 Schematic of bar testing results for single bar batch quench, and multi-bar batch quench 4330V Furnace type Continuous bar rotation Pit required Throughput Maintenance Bar marking In-line tunnel Yes No Low Low Low Walking beam Partial Yes High High Low Rotary screw hearth Yes No High Low Partial r Table 1 Furnace type comparison suffered from warpage resulting in straightening after Q&T. In addition to increasing (work in process) (WIP) inventory, this adds some $30-40/t in cost to the finished steel product. Since bar to bar contact does exist for such systems, the non-uniformity of quench around the circumference of the bar can lead to inconsistent microstructure and mechanical properties. Figure 2 illustrates bars emmersed in the quench medium PAG (Polyal Kalene Glycol). Figure 3 shows the wide range in mechanical properties for 305mm rounds that were batch immersion bundle quenched vs bars that were single bar immersion quenched. Mechanical and metallurgical testing at positions around the circumference (12, 3, 6 and 9 o clock) clearly show that areas of reduced quench uniformity resulted in low Charpy impact values, and were confirmed by a lower bainitic structure. This highlights one of the main drawbacks of any quenching technology where the circumference of the bar along its length is not uniformly quenched. Uneven cooling during the quench leads to different transformation times for change in austentite to martensite, resulting in uneven volumetric changes and hence distortion. To alleviate these effects, a state-of-the-art bar heat treatment technology would have to have the following features: ` Single bar uniform quenching technology ` Continuous bar rotation during austenitise and temper cycles ` Multiple zones of control for both austenitise and temper furnaces ` High volume capacity systems HIGH VOLUME CONTINUOUS Q&T FURNACES Since both batch immersion quenching and continuous roller hearth-restrained quenching have many inherent limitations (low throughput, non-uniform properties), and they produce bars of unacceptable as-quenched and tempered straightness, furnace builders sought technologies that could produce commercially straight bars direct from the equipment without post straightening. Since the main aspect of consideration for the furnace equipment is continuous bar rotation during both the austenitising and tempering portions of the cycle, there are a limited number of continuous furnace systems that satisfy this requirement. Continuous bar rotation during the austenitising phase is critical since it actually improves the incoming bar straightness, and provides a selfstraightening feature. The main continuous furnaces that would satisfy the needs for continuous bar rotation are the in-line tunnel furnace, walking beam furnace and rotary screw hearth furnace. These are compared in Table 1. The only true systems to achieve continual bar rotation during heating prior to quench, are in-line tunnel furnaces (Figure 4) and rotary screw hearth furnaces (Figure 5). The walking beam furnace system (Figure 6) achieves a partial rotation during every cycle when the furnace indexes forward one pitch position. Dependent on the beam design, this could leave the bar in a simply supported load condition for most of the cycle, and creep is a concern as austenitising temperature is approached. The in-line tunnel furnace system utilises angled v-groove or hourglass shaped rolls that convey a single bar through a heated chamber. Production rates are relatively low and range in the 2-6t/hr capacity. Some units employ induction 156
FORMING PROCESSES preheating to boost throughput, but this presents a host of other problems (dummy bars, etc). Rotary screw hearth austenitise and temper furnaces have some clear advantages over other continuous furnace systems (eg, high volume, no foundations, ease of maintenance and simple conveyance method). The only negative comment on the technology is that the screws themselves tend to put some superficial marks on the bars. Since nearly all the bar stock is destined for some post heat treatment machining, either at the mill source or at the distribution level, these marks clean up with a light skin pass (typically 1.5mm). With the rotary screw hearth furnace concept, bars are conveyed on refractory piers by high alloy cast screws that utilise a proprietary connection method. The screws are fully supported in refractory troughs and do not run the risk of deflection with exposure to high temperatures. Experience with such systems has shown the screw life to be in excess of 15 years, with minimal wear. Multiple screws are employed in a clockwise and counterclockwise rotation method, ensuring that bars stay centred during their conveyance. Production throughput for a typical continuous plant is ~60-120kt/yr and is capable of processing bar diameters in the range of 20-305mm. Special consideration is required to ensure the bar is centred in the spray quenching tunnel, otherwise crooked bars will result. Although there is a tendency for the buyer to specify equipment that covers a very broad range of bar diameters, serious consideration should be given to developing an optimum range of diameters, since production output at the low end of the spectrum (below 75mm) can produce very low outputs. r Fig 4 In-line tunnel furnace r Fig 5 Rotary screw hearth furnaces ALLOYS FOR Q&T Virtually all carbon and alloy steels can be run through a high volume continuous steel bar Q&T system. This would cover most steels that are hardened by conventional immersion water or oil methods. The limitation would be higher alloy steels that are capable of developing their properties via air quenching (eg, EN30B ~0.3C, 1.4Cr, 4.0Ni, 0.2Mo). In such cases the water spray quench could be bypassed to achieve the air quench. Generally speaking, some resulphurised alloy steels (eg, 4150S) are not suitable for spray quench since the high distribution of manganese sulphides (MnS) can cause the bar to split along its longitudinal axis creating a catastrophic failure of the quenching system. r Fig 6 Walking beam furnace Regardless of the continuous furnace conveyor system, the heart of the heat treatment system is the spray quench technology. This consists of a v-groove roller conveyor, and a series of quench rings with a specific nozzle pattern to a SPRAY QUENCHING TECHNOLOGY 157
r Fig 7a and 7b Effect of water flux intensity on cooling rate cool the bars rapidly from austenitising temperature to below the M f (martensite finish) temperature. The desired exit quench bar temperature is between 100 and 200 C, depending on the alloy composition which dictates the M f. Spray quenching technology offers many advantages over conventional immersion technology, including: Operation parameter Water temperature Water pressure Water velocity Bar diameter Influence on the heat transfer coefficient ` Single bar quenching allows for uniform mechanical and metallurgical properties ` Water flux intensity and water temperature can be varied to produce a wide range of quenching characteristics with equivalent cooling rates to immersion oil or immersion polymer quenching ` Water is recirculated, environmentally friendly, poses no fire risks (unlike quench oils), and is an abundant cost-effective quench ` Water flux intensity continually breaks up the vapour barrier blanket formed in conventional quenching without the use of costly mechanical agitators (impellers) ` Production of stress-free bars that do not require post Q&T straightening Figures 7a and 7b show the effect of water flux intensity on cooling rate. From these curves it can be clearly seen that an increase in water flux intensity, defined as litres/ min/m 2, has a significant impact on the cooling rates of steel bars in a spray quenching application. One of the other main considerations is the water temperature. Generally an increase in water temperature produces a lower cooling rate. Typically, the following considerations can be made when summarising the key variables that affect spray quenching technology. One item not mentioned is the effect of scale (iron oxide) on the bar surface at the time of quenching, which can limit the heat transfer through the scale boundary layer. In some tube and pipe quenching r Table 2 Key operation parameters and their effect on the HT coefficient applications, descaling prior to quench is necessary to achieve the mechanical properties for this reason. DEFECTS IN SPRAY QUENCHING TECHNOLOGY Spray quenching can produce a quench ring crack, typically on bar diameters above 100mm, or on high carbon steels (>0.55C). The reason for this phenomenon is that the ends of the bars pose a discontinuity in the quenching boundary conditions and so rapid quenching exists in both the longitudinal and transverse directions. For larger diameter bars, or steels with a higher quenched hardness, this sets up a significant stress gradient in these two orientations that can produce a quench ring. The resultant yield loss can be approximately 25mm off each end of the bar. Some work has been done to introduce a soft cooling pattern in the initial quench locations, and all indications are that this has decreased the frequency and severity of such defects. The conveyance method during austenitising appears to have little, if any, effect on the nature or appearance of ring cracks (see Figure 8). LEVEL 2 AUTOMATION State-of-the-art quenching systems employ level 2 automation systems to perform the following functions. They allow for a fully automated line and all processing parameters are downloaded to the equipment. Examples include: 158
FORMING PROCESSES ` Bar tracking through the furnace ` Line speeds and processing parameters ` Recipe control ` Alarm functioning ` Process history database ` Maintenance notification An example computer screen print is shown in Figure 9. CONCLUSIONS Stress-free bar production is a reality that is possible only through the proper design of both the continuous industrial furnace, and the associated water spray quench technology. The careful combination of these two factors produces high volume steel bar Q&T production without the need for post heat treatment straightening. The benefit to the steelmaker is a lower overall operating cost per tonne produced. The end user of the pre-hardened bars has the added benefit of stress-free bars that will not warp during machining operations. MS r Fig 8 Ring cracks Michael K Klauck is Product Manager Custom Products and Gregory R Stanley is Furnace Engineer, both at Can-Eng Furnaces International Ltd, Niagara Falls, Ontario, Canada. CONTACT: mklauck@can-eng.com r Fig 9 Layout of continuous bar heat treatment system 159