Heat-treating processes are designed to give metal components a microstructure that is able to withstand any loading conditions that arise from the function of the component. One step in the process of controlling the component’s microstructure is to quench (cool) the part from the heat-treatment temperature to a specified lower temperature under controlled manufacturing conditions. The quenching bath removes heat from the metal being hardened at a rate that is faster than the critical cooling rate. The more rapid the rate of heat extraction above the cooling rate, the higher the resulting hardness. Proper quenching produces components with oversaturated microstructures, which are the starting points for age hardening or tempering.

Polyalkylene glycol (PAG) products have a proven record of superior performance and offer significant advantages over water, oil or other types of polymer quenchants. When compared to conventional polymer fluids, these PAG solutions are shear and hydrolytically stable, and they offer less sensitivity to hard water and iron-oxide scale. In addition, residue that is normally deposited during the quenching process is more easily removed from the workpiece. PAG quenchants also provide excellent quench uniformity, less variation in quenchant intensity as baths age, moderate biodegradability and the ability to clean the quenchant bath by thermal separation.

PAG-based quenchants are versatile products, and they can be tailored to meet a wide variety of heat-treating requirements through selection of quenchant type, quenchant concentration, quench-bath temperature and quench-bath agitation rate.

Quenchants Put to the Test

To confirm that PAG-based quenchants would provide improved quenching performance while also improving productivity, three manufacturers in China collaborated with Dow to initiate quenchant tests within their heat-treating operations. Four tests were conducted at the major steel manufacturer, while single experiments were conducted at a high-strength fastener company and a machinery company.

The tests involved two quenchants: UCON Quenchant A and UCON Quenchant E. Typical physical properties of the quenchants are shown in Table 1.

Figure 1 illustrates cooling time and cooling-rate curves for the two quenchants at 5% concentration. UCON Quenchant A is considered to be a relatively “fast” quenching product. Performance of UCON Quenchant E is considered similar to oil-based quenchants.

All tests were conducted under controlled conditions observing safety requirements as specified in current Material Safety Data Sheets for the quenchants and the User’s Manual.

Components used in the tests included die blocks and a variety of metal parts in commonly used alloys, including P20; 718; 5CrNiMo; 25CrNiMo; M8, M10, ML35 and K35 bolts and nuts; 35 and 42 CrMo; and 40Cr alloy. Typical die-block quenched surface hardness ranged from 28-36HRC. The quenched internal hardness ratings were from 4-5HRC.

Equipment used in the experiments with the major steel manufacturer included a quenching bath with an estimated 130-140MT (metric ton) capacity and two agitators installed inside the bath. Agitator speed was set at 0.2–0.4m/s. The quenching bath was also fitted with filtration, IHT, thermometer and additional safety measurement devices.

Test #1 – Steel Manufacturer
Die blocks of P20 Alloy measuring 510 x 1350 x 3000mm and weighing 16 tons were selected for the first trial. Chemical properties of the alloy are shown in Table 2.

Fig. 2. As-quenched hardness of die block achieved with bath in a 15% solution of UCON Quenchant E

The die blocks were heated to 860°C (1580°F), then quenched in a 45°C bath of 15% UCON Quenchant E for three hours (Fig. 2). Die blocks were subsequently reheated at 560°C (1040°F). The resulting as-quenched surface hardness was 30-32HRC (Fig. 2), with an as-quenched internal hardness of 3HRC.

Test #2 – Steel Manufacturer
Die blocks of 718 Alloy measuring 660 x 1260 x 2520mm and weighing 16.5 tons were selected for the second experiment. Chemical properties for this material are shown in Table 2.

The die blocks were heated to 860°C (1580°F), then quenched in a 25°C bath of 23% UCON Quenchant E for five hours. Die blocks were subsequently reheated at 550°C (1020°F). Heat treatment achieved an as-quenched surface hardness of 32.7-34.3HRC (Table 3) and an as-quenched internal hardness of 2HRC.

Test #3 – Steel Manufacturer
Blocks of 5CrNiMo Alloy measuring 310 x 315 x 2500mm and weighing 2 tons were selected for the third experiment. Chemical properties for the metal are shown in Table 2.

The die blocks were heated to 860°C (1580°F), then quenched in a 44°C bath of 15% UCON Quenchant E for 70 minutes. The temperature was estimated at 70°C upon removal from the quenchant bath and projected to rise to ~100°C within five minutes time. The die blocks were subsequently reheated to 450°C (842°F). Heat treating resulted in an as-quenched surface hardness of 40HRC and an as-quenched internal hardness of 6.5HRC. There was no evidence of breakage or cracking after cooldown to 50°C.

Test #4 – Steel Manufacturer
Complex metal parts of 42CrMo and 25CrNiMo were selected for the fourth trial. Chemical properties for the 25CrNiMo components are shown in Table 4.

The components were heated to 890°C (1630°F), then quenched in a 45°C bath of 15% UCON Quenchant E for 2.5 hours. The part temperature was estimated at 60°C after removal from the quenchant bath. Components were subsequently reheated twice to 630°C (1165°F) and 650°C (1200°F). As-quenched surface hardness was measured at 220-234HB, and there was no evidence of breakage or cracking of components.

Fig. 3. Bolts and nuts are fed into the continuous-carburizing quenching furnace

High-Strength Fasteners

This fastener producer operates lines of quenching furnace belts that process nuts and bolts (Fig. 3). The experiment was conducted on furnace belt #6, which operates at 1MT/hour. The 12MT-capacity quenching baths include three loading stations (high, medium and low) and are equipped with spraying capability. Quenching-bath temperature is maintained at >50°C and is monitored by IHT. Bolts and nuts of Alloy M8, M10, ML35 and K35 were heat treated under controlled conditions between 600°C (1110°F) and 300°C (570°F) to prevent distortion.

Parts were quenched in a 10-15% solution of UCON Quenchant E for one hour. Officials from the fastener company noted that UCON Quenchant E provided a faster cooling rate than oil at this temperature. Carburizing control was maintained to within 0.3%-0.35% in the whole belt. As-quenched surface hardness ranged from 50HRC-52.5HRC, and the reheat as-quenched surface hardness ranged from 24.5HRC-26.0HRC. Reheat as-quenched internal hardness was <2HRC. Hardening met the mandatory standard of 8.8 scale, and there was no observable cracking or distortion in the components.

Fig. 4. Quenching investment casting sheaves in IHT

Machinery Production

Fine-casting sheaves used in machinery production were heat treated and cooled in a bath of UCON Quenchant A. The sheaves – constructed of 35CrMo, 42CrMo and 40Cr Alloy – measured 250mm and 600mm in length, with a radius of 200mm and 400mm respectively (Fig. 4). Quenching-bath dimensions were 5.0m x 3.0m x 4.0m, with a capacity of up to 50MT of aqueous solution. The water flow rate was 0.3-0.4m/s, bath temperature was set at 30-50°C and light agitation was applied during the quenching process. Prior to quenching, the sheaves were heated to between 840°C (1540°F) and 860°C (1580°F) and reheated to 560°C (1040°F) following quenching. All surface hardness of metal alloys met product specifications, and there were no incidents of breakage or cracking. Results for this test are summarized in Table 5.

Conclusions

The results of the six tests show that PAG-based quenchants can provide China’s heat-treating industry with improved quality and performance in a wide range of metal-treating applications. In addition to quenching performance, PAG-based quenchants offer other advantages over alternative quenchant technologies. They are especially good alternatives to oil quenchants because they are non-flammable, eliminating the danger of fire and the creation of smoke during heat-treating operations.

PAG-based quenchants typically cost less than oil and offer increased environmental protection because spills are easily cleaned up. Because multiple quenching speeds may be obtained from the same product, PAG quenchants can provide greater flexibility in plant operations while reducing processing costs.

PAG-based quenchants are also superior to water as a quenching medium. Water exhibits non-uniform heat transfer due to vapor-blanket (A-stage) instability, which can cause thermal gradients that can cause high residual stress and distortion. In contrast, PAG-based quenchants stabilize the vapor blanket to provide uniform cooling and reduce the potential for cracking and distortion.IH

For more information:For additional information about UCON Quenchants, including assistance in selecting and using UCON Quenchants in specific applications, contact The Dow Chemical Company in Asia: tel: +800 7776 7776 or +60 3 7958-3392; or in N. America: 800-447-4369; web: www.ucon.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: polyalkylene glycol, cooling rate, quench bath, thermal gradient

SIDEBAR: About UCON™ Quenchants

UCON Quenchants are a series of PAG-based, non-flammable, aqueous solutions containing special polymers and corrosion inhibitors for quenching both ferrous and nonferrous metals.

Included is UCON Quenchant E, which is the most “oil-like” quenchant in the line. It is based on a patented polymer structure and is used to replace medium-slow quench oils. It is especially effective for high-carbon/high-hardenability steels and for quenching thin aluminum sheets.

Also included is UCON Quenchant A, which can be used to replace fast quench oils and is especially effective for induction heat treating, low-carbon and low-hardenability steels, and integral-quench furnaces. UCON Quenchant A is also the original PAG quenchant approved for use in heat treating aluminum for aerospace applications, and it meets all requirements for a Type 1 quenchant as specified by AMS 3025B.