In the demanding world of industrial machinery, the longevity and reliability of key components are paramount. Many of these critical pieces are large, complex casting parts manufactured from wear-resistant steels and irons. The quest for optimal mechanical properties in these massive casting parts often leads to a significant heat treatment challenge: achieving the desired hardened microstructure without inducing catastrophic cracking during quenching. For decades, the solution to this dilemma for large forgings and castings has been a technique known as Alternated Timed Quenching by Water and Air, or ATQ. This process represents a masterful application of metallurgical principles to practically control cooling, marrying the intense cooling power of water with the gentle respite of air in a precisely timed sequence. My experience and research confirm that the ATQ process is not merely an alternative but often the *only* viable method to successfully heat-treat large-section wear-resistant casting parts to their full potential.
The core challenge stems from the fundamental conflict between hardenability and quench cracking susceptibility. Wear-resistant casting parts, such as mill liners, grinding rods, and crusher hammers, require a hard microstructure—typically martensite, often with secondary carbides—to resist abrasion. Achieving this necessitates a cooling rate during quenching that exceeds the critical cooling velocity (Vc) specific to the alloy’s composition. For thick-section casting parts, oil quenching frequently fails to provide sufficient cooling speed to avoid the formation of softer transformation products like pearlite or bainite in the core. Conversely, direct water quenching provides ample cooling power but introduces severe thermal and transformational stresses, making the casting parts highly prone to cracking, especially as they undergo the martensitic transformation.
The ATQ process ingeniously navigates this narrow window. It is a controlled, iterative cooling method where the heated casting part is alternately immersed in water and exposed to air. The timing of each phase is not arbitrary but is meticulously designed based on the steel’s Time-Temperature-Transformation (TTT or C-curve) diagram and its Martensite Start (Ms) temperature. The strategic objective is twofold: first, to ensure cooling through the pearlite and bainite “noses” of the C-curve is fast enough (using water) to avoid their formation; and second, to slow down the cooling precisely through the Ms to Mf (Martensite Finish) temperature range (using air) to mitigate the stresses of the martensitic transformation. The process can be summarized by a critical temperature control function:
$$ T_{quench}(t) = \begin{cases}
T_{initial} – \beta_w t & \text{for } t \leq t_w \text{ (Water immersion)}, \\
T_{water-exit} + \alpha_a (t – t_w) & \text{for } t_w < t \leq t_a \text{ (Air exposure)}, \\
… & \text{Cycle repeats as needed}
\end{cases} $$
Where $T_{quench}(t)$ is the part’s surface temperature over time, $\beta_w$ is the average cooling rate in water, $\alpha_a$ is the average heating rate in air (due to residual core heat), and $t_w$, $t_a$ are the meticulously controlled times in each medium. The goal is to keep $T_{quench}(t)$ near or slightly below Ms during the air phases to allow a slow, controlled transformation.
The foundation of any successful ATQ protocol is a deep understanding of the material’s C-curve. For wear-resistant casting parts, we typically deal with three broad categories of transformation behavior, each demanding a tailored ATQ strategy. The first category includes medium-carbon, low-alloy steels. Their C-curves have a distinct “nose” for pearlite transformation. The ATQ focus here is a rapid water quench from the austenitizing temperature down to a point just above the Ms, followed by an air hold to allow temperature equalization and slow transformation onset. The second category encompasses many alloy wear steels, where the C-curve is shifted to the right, introducing a pronounced bainite region. The ATQ sequence must ensure the cooling path skirts the bainite nose, often requiring a specific water quench to a temperature between the pearlite and bainite noses before the first air cycle. The third category includes high-alloy materials like some white irons, where the high hardenability pushes the pearlite nose very far to the right. Here, the primary challenge is not avoiding pearlite but managing the stress of transforming a high-carbon austenite matrix; the ATQ process is finely tuned to the Ms point.
The cornerstone of process design is the Ms temperature. While precise measurement is ideal, it can be estimated for steel casting parts using empirical formulas based on chemical composition. One widely used approximation is:
$$ M_s (^\circ C) = 539 – 423C – 30.4Mn – 17.7Ni – 12.1Cr – 7.5Mo $$
(Where C, Mn, etc., are the weight percentages of the elements.)
For high-chromium white cast irons, the Ms is significantly lower, often in the range of 200°C to 280°C, and is heavily influenced by the austenite’s carbon and chromium content. The first water quench duration ($t_w1$) is calculated to cool the part from the austenitizing temperature ($T_A$) to a target temperature ($T_{target}$) just above Ms. This requires knowledge or estimation of the heat transfer coefficient ($h$) in the agitated water quench tank. The time can be approximated using the lumped capacitance method for simpler geometries, though for complex casting parts, empirical data from trials is invaluable:
$$ t_w1 \approx \frac{\rho C_p V}{h A_s} \ln \left( \frac{T_A – T_{water}}{T_{target} – T_{water}} \right) $$
Where $\rho$ is density, $C_p$ is specific heat, $V$ is volume, $A_s$ is surface area, and $T_{water}$ is the water bath temperature. The subsequent air time ($t_a$) is set to allow the core heat to diffuse to the surface, raising the temperature back towards Ms but not exceeding it, to prevent any undesired high-temperature transformation. This cycle may be repeated multiple times for very thick sections or high-hardenability steels to gently guide the entire part through the martensitic transformation zone.
It is imperative to understand that the success of the ATQ process is intrinsically linked to the quality of the casting parts themselves. No heat treatment can salvage a fundamentally flawed casting. The process demands casting parts with high metallurgical purity: low levels of harmful trace elements like sulfur and phosphorus, minimized non-metallic inclusions, and a refined, uniform grain structure. Macro-defects such as shrinkage cavities, porosity, or cold shuts act as potent stress concentrators and will almost certainly lead to crack initiation during the thermal cycling of ATQ. Therefore, a robust casting process involving proper gating, risering, inoculation, and melting practice is the non-negotiable prerequisite for implementing ATQ on wear-resistant casting parts.
The practical application of ATQ spans a diverse range of wear-resistant alloys. The following case studies, presented with key data in tabular form, illustrate its versatility and critical parameters.
Case 1: Medium-Alloy Steel Mill Liners (e.g., ZG35Cr2Si2CuMo)
This alloy is designed for impact-abrasion service in ball mills. The goal is a martensitic matrix with high hardness and good toughness. Direct water quenching of these large, uneven-section casting parts leads to cracking, while oil quenching fails to achieve the target hardness in the core.
| Parameter | Value/Specification |
|---|---|
| Typical Casting Part | Wet Ball Mill Liner (~900x470x151 mm, ~265 kg) |
| Austenitizing Temperature | 950 – 980°C |
| ATQ Sequence | Water quench: 275 s → Air: 25 s → Final Water: 5 s |
| Quench Water Temperature | 10 – 40°C |
| Target Exit Temperature | 80 – 140°C (before final transfer to tempering furnace) |
| Resulting Hardness (HRC) | 54 – 56 |
| Impact Energy | > 25 J (unnotched) |
| Microstructure | Martensite |
The initial long water quench ensures cooling past the pearlite nose. The brief air cycle allows stress relaxation and temperature equalization just as the surface approaches Ms. The final short water dip completes the cooling to a safe temperature for handling and tempering. This protocol consistently produces crack-free, high-performance liners.
Case 2: High-Carbon Chromium Steel for Semi-Autogenous (SAG) Mill Liners (e.g., ZG80Cr2MoNi)
These massive casting parts (often over 1 ton) require a tempered sorbitic (fine pearlitic) structure for a combination of strength, toughness, and wear resistance—a classic调质 (quench and temper) application. Their size makes oil quenching ineffective and water quenching far too drastic.
| Parameter | Value/Specification |
|---|---|
| Casting Part | SAG Mill Shell Liner (~1300 kg) |
| Heat Treatment Goal | Quench & Temper (调质) for Sorbitic structure |
| Austenitizing Temperature | 860 – 890°C |
| ATQ Method | Single, controlled water quench to ~450°C |
| Subsequent Step | Immediate transfer to tempering furnace at 450°C, hold, then heat to final tempering temperature (e.g., 600-650°C) |
| Final Hardness (HRC) | 32 – 40 |
| Microstructure | Tempered Sorbitte + Carbides |
Here, the ATQ process is effectively an interrupted quench. The part is water-cooled just fast enough to miss the pearlite nose but is removed from the water at a high temperature (~450°C), well above Ms. It is then immediately placed in a furnace at that temperature. This allows the isothermal transformation of austenite to a fine, non-lamellar transformation product (like bainite) or prevents further cooling, after which the part is tempered. This avoids the high stresses of martensite formation altogether while achieving the desired strength.
Case 3: Grinding Rods for Rod Mills
Long, cylindrical casting parts like grinding rods are prone to bending and distortion during quenching. A continuous ATQ process in specialized handling equipment is the ideal solution.
| Material | Rod Diameter (mm) | Austenitize Temp. (°C) | Water Quench Time (s) | Target Exit Temp. (°C) | As-Quenched Hardness (HRC) |
|---|---|---|---|---|---|
| 42CrMo | 90 | 860 | 130 | 120-170 | ~55 |
| 65Mn | 90 | 780 | 130 | 120-170 | ~53 |
| GCr15 | 85 | 860 | 120 | 120-180 | ~63 |
| High-Cr Steel (Cr10) | 110 | 1000 | 154 | 240-300 | ~58 |
Rods are austenitized in a continuous pusher furnace and rolled into a shaking water tank. The controlled quench time is precisely calculated based on diameter. The rods are then discharged onto a bed where residual heat provides a “self-tempering” effect, often eliminating the need for a separate low-temperature furnace cycle. This is a highly efficient, mass-production application of the ATQ principle.
Case 4: High-Chromium White Cast Iron Crusher Hammers
These highly abrasion-resistant casting parts have a high alloy content, moving their pearlite nose far to the right. The primary challenge is preventing cracks during the formation of hard, brittle martensite in the matrix. The Ms for these materials is very low.
| Parameter | Value/Specification |
|---|---|
| Material | High-Cr White Iron (e.g., ~3.1%C, 23%Cr) |
| Casting Part | PF1210 Impact Crusher Hammer (~200 kg) |
| Ms Point | ~280°C |
| Austenitizing Temperature | 980 – 1020°C |
| ATQ Sequence | Water quench for 120 s to ~500°C → Air cool to Ms (~280°C) |
| Final Step | Immediate transfer to tempering furnace at 200-250°C |
| Achieved Hardness | > 61 HRC (after tempering) |
The water quench to 500°C is fast enough to avoid the greatly delayed pearlite transformation. The subsequent slow air cooling through the critical Ms to Mf range is the key to preventing cracking. The immediate low-temperature tempering relieves stresses and stabilizes the microstructure. The process yields a crack-free part with a microstructure of martensite, primary (M7C3) carbides, and some retained austenite, delivering exceptional abrasion resistance.
In conclusion, the ATQ quenching process is a powerful and essential tool in the heat treatment arsenal for large, high-value wear-resistant casting parts. It transcends the limitations of conventional single-medium quenching by offering unparalleled control over the cooling path. By strategically alternating between vigorous water cooling and gentle air cooling, guided by the material’s TTT diagram and Ms point, it successfully reconciles the conflicting demands of deep hardenability and low quench stress. From massive SAG mill liners requiring a tough, tempered structure to intricate high-chromium iron components demanding extreme hardness, the ATQ process proves its versatility and reliability. Its successful implementation, however, rests on a tripod of precise metallurgical knowledge, meticulous process timing, and, fundamentally, the sound quality of the underlying casting parts. For engineers and metallurgists tasked with maximizing the service life of critical machinery components, mastering the ATQ process is not just an advantage—it is often the definitive step towards achieving performance and durability that standard methods cannot reach.