Tempering Temperature and Drilling-Tool Requirements
As exploration projects reach greater depths and reaming diameters increase, the loads carried by drilling tools become both larger and more complex, and the string is expected to absorb shock loading as well as transmit torque. The material chosen for these tools is normally a high-hardenability alloy structural steel that is quenched and tempered rather than used in the as-rolled condition, because only a tempered structure combines the high strength needed to resist bending and torsion with enough plasticity and toughness to survive impact.
Wear-resistant plate is an alloy wear-resistant steel of exactly this type. After quenching and tempering it reaches high strength while retaining good plasticity and toughness, which is why it is widely used in the production of large-diameter drilling tools. For the mining and exploration duty considered here, the mechanical property targets are demanding and specific:
Tensile strength above 1033 MPa
Hardness higher than 321 HBW
Longitudinal impact absorption energy KV2 not less than 90 J
Reaching all three at once is the difficulty: hardness and tensile strength usually rise together with falling tempering temperature, while impact energy falls. This study examined how different high temperature tempering temperatures affect the microstructure and the mechanical properties of wear-resistant plate, so that the tempering temperature can be selected against the actual service requirement rather than by habit.
Material, Heat Treatment and Test Programme
The experimental material was the residual (offcut) section of a large annealed wear-resistant plate used in petroleum machinery, cut to 60 mm × 60 mm × 125 mm with an initial grain size of ASTM 4.0. Its chemical composition is given below in weight percent.
| Element | C | Si | Mn | P | S | Cr | Ni | Mo | V |
|---|---|---|---|---|---|---|---|---|---|
| wt % | 0.333 | 0.212 | 0.559 | 0.0054 | 0.0015 | 0.977 | 2.45 | 0.469 | 0.0902 |
All specimens were austenitised at 880 °C for 2 h and oil quenched, then tempered at 560, 590, 620 and 650 °C respectively, each for 2 h followed by water cooling. This is the range in which a quenched alloy structural steel is normally subjected to high temperature tempering, and it brackets the tempering temperature expected to give the best strength and toughness combination.
Sampling: tensile and impact specimens cut at the corresponding position of the bar in accordance with GB/T 2975
Tensile testing: in accordance with GB/T 228
Impact testing: longitudinal specimens in accordance with GB/T 229, reported as KV2
Hardness: Brinell method in accordance with GB/T 231.1, 10 mm ball, 29.4 kN test force, 15 s load dwell
Microstructure: specimens ground, polished and etched in 3 % nital for 10 to 15 s, examined on an inverted metallographic microscope
Microstructure After Quenching and Tempering
After quenching from 880 °C and tempering anywhere in the 560 to 650 °C range, the microstructure of the wear-resistant plate consists of tempered sorbite together with flake or blocky ferrite and dispersed carbide. That structure is the direct result of the tempering treatment: the as-quenched martensite decomposes, the carbides precipitate and coarsen, and the ferrite matrix recovers progressively as the temperature rises.
The change across the range is one of degree rather than of type. At the low end of the range the carbide dispersion is fine and the ferrite is heavily substructured, which delivers maximum strength and hardness. At the high end the carbides are coarser and more widely spaced and the ferrite is softer and more recovered, which lowers strength and hardness but allows the material to absorb far more energy before fracture. Because no new phase appears, the whole range remains a single, predictable response curve that can be used to tune the product.
Strength, Hardness and Impact Response
The measured trends across the four tempering temperatures are summarised below. Strength and hardness decrease monotonically as tempering temperature increases, while longitudinal impact absorption energy rises.
| Tempering temperature | Tensile strength and yield strength | Brinell hardness | Impact absorption energy (KV2) | Overall balance |
|---|---|---|---|---|
| 560 °C | Highest of the range | Highest, above the 321 HBW requirement | Lowest of the range | Strength-led; toughness margin is smallest |
| 590 °C | Lower than 560 °C | Below the 560 °C level | Higher than 560 °C | Intermediate |
| 620 °C | Still above the 1033 MPa requirement | Above the 321 HBW requirement | Above the 90 J requirement | Best strength and toughness matching |
| 650 °C | Lowest of the range | Lowest of the range | Highest of the range | Toughness-led; strength margin is smallest |
Two results define the useful window. After tempering at 560 °C the room temperature strength and Brinell hardness reach their maximum while the measured impact absorption energy is at its lowest - a combination that satisfies the strength requirement with the least protection against shock loading. After tempering at 650 °C the opposite is true: the material is at its most tolerant of impact but at its weakest. Only at 620 °C does the steel simultaneously clear the 1033 MPa tensile strength, 321 HBW hardness and 90 J impact energy targets with margin on all three.
Selecting the Optimum Tempering Temperature
Taking strength and plasticity together, 620 °C is the optimum tempering temperature for wear-resistant plate used as a large-diameter special drilling tool material for mining and exploration. The selection is not merely a compromise between two competing properties; it is the point at which the tempering response of this composition places all three acceptance criteria comfortably inside specification.
Use 620 °C for 2 h with water cooling as the standard temper for large-diameter drilling tool plate, and confirm the result with tensile, impact and Brinell tests on every heat-treated batch.
Move to 560 °C only where the duty is dominated by strength and hardness and where shock loading in service is known to be low.
Move towards 650 °C only where impact toughness governs the design and the reduced strength and hardness still satisfy the tool calculation.
Treat tempering temperature as a controlled variable with the same discipline as austenitising temperature; a modest drift changes hardness and impact energy in opposite directions and can take the product out of specification on one property while the other improves.
Verify the microstructure as well as the mechanical values, since tempered sorbite with flake or blocky ferrite and carbide is the structure that corresponds to an acceptable, fully tempered product.
FAQ
Q: What tempering temperature is best for wear-resistant drilling tool plate?
620 °C for 2 h followed by water cooling. It is the only temperature in the 560 to 650 °C range that meets the tensile strength, hardness and impact energy requirements at the same time.
Q: What microstructure forms after quenching at 880 °C and tempering?
Tempered sorbite with flake or blocky ferrite and dispersed carbide, throughout the 560 to 650 °C tempering range.
Q: How do the mechanical properties change with tempering temperature?
Tensile strength, yield strength and Brinell hardness fall steadily as the temperature rises, while longitudinal impact absorption energy rises steadily.
Q: What happens after tempering at 560 °C?
Room temperature strength and Brinell hardness reach their maximum and impact absorption energy is at its minimum, giving the smallest toughness margin.
Q: What are the property targets for this application?
Tensile strength above 1033 MPa, hardness above 321 HBW and longitudinal impact absorption energy KV2 of at least 90 J.
Q: How should the product be verified after tempering?
Cut tensile and impact specimens to GB/T 2975 and test them to GB/T 228 and GB/T 229, measure Brinell hardness to GB/T 231.1 with a 10 mm ball at 29.4 kN, and check the metallographic structure.
