By Harris, Technical Staff, SHUNFU METAL
Laser quenching process sheets usually promise one number: a single hardness, a single speed, a single power. The 2021 study by Liang Chenfan et al. (Transactions of Materials and Heat Treatment, 42(6):147–156) on 4145H drilling tool steel shows why that mindset is wrong. With a 24-run matrix spanning scan speeds from 100 to 800 mm/min and powers from 1.5 to 2.2 kW, the paper proves that hardness and wear resistance follow a bell curve: they rise, peak, and fall. Too fast fails. Too slow fails just as badly — the surface overheats past the melting threshold and the phase transformation never happens. The practical takeaway for drill tool buyers: the window, not the peak, is what you are paying for.
4145H and Its International Grade Family
4145H is the AISI chromium-molybdenum H-steel of the oilfield, registered as UNS G41450 and supplied to ASTM A29/A304; the H suffix guarantees a hardenability band, which is why it is the designated material for drill collars and heavy-weight drill pipe in API Specification 7-1. In China the grade is produced for the petroleum drill-collar standard SY/T 5144, and buyers routinely cross-reference the domestic GB/T 3077 substitute 42CrMo. The nearest EN counterpart is 42CrMo4 (W.Nr. 1.7225), with the same family appearing as JIS SCM440 in Japan. One caveat worth repeating: 42CrMo4 carries less carbon (0.38–0.45% versus 4145H’s 0.42–0.49%), so its laser-quench window is not identical. The 2021 study tested a drill-collar-tightened 4145H whose nominal chemistry is shown below.
| C | Si | Mn | P | S | Cr | Mo | Fe |
|---|---|---|---|---|---|---|---|
| 0.43–0.47 | 0.20–0.30 | 1.00–1.10 | ≤0.02 | ≤0.02 | 1.10–1.20 | 0.20–0.25 | Bal. |
The Old Assumption: Slower Is Always Better — and Why It Broke
The companion 2019 study on the same grade tested only 400, 600 and 800 mm/min and concluded that lower speed consistently wins. The 2021 team did the honest thing: they extended the window downwards to 100, 200 and 300 mm/min. The result is a correction that matters commercially. Hardness and wear resistance increase first and then decrease as both scan speed and laser power rise. In plain terms: at the slow end the surface is heated above the 1600 °C melting threshold adopted in the study, the molten-overheated layer loses its self-cooling margin, and the structure never transforms into martensite; at the fast end there is not enough heat for transformation either. Only the middle produces the fine, wear-resistant case.
The 24-Run Matrix
A CO₂ laser with a φ10 mm spot scanned each specimen in two overlapping tracks. Power levels were 1.5, 1.8, 2.0 and 2.2 kW; speeds were 100, 200, 300, 400, 600 and 800 mm/min. Surface hardness was measured on the HRC scale, and wear was tested on an MMW-1 ball-on-disk machine (4 mm steel ball, 100 N, 200 r/min, 30 min) with the worn surfaces examined by scanning electron microscopy.
| Power (kW) | 100 mm/min | 200 mm/min | 300 mm/min | 400 mm/min | 600 mm/min | 800 mm/min |
|---|---|---|---|---|---|---|
| 1.5 | 1 | 2 | 3 | 4 | 5 | 6 |
| 1.8 | 7 | 8 | 9 | 10 | 11 | 12 |
| 2.0 | 13 | 14 | 15 | 16 | 17 | 18 |
| 2.2 | 19 | 20 | 21 | 22 | 23 | 24 |
Speed: Too Slow Melts, Too Fast Never Transforms
At 2 kW, the microstructure map across speed is a complete story. At 100 and 200 mm/min the hardened layer is blocky ferrite and blocky pearlite — the surface temperature overshoots the melting threshold, overheating destroys the self-cooling margin, and no martensite forms at all, leaving low hardness and severe wear. At 300 mm/min the structure becomes granular bainite with retained austenite, a step up in hardness. At 400 mm/min fine acicular martensite appears and hardness peaks. At 600 mm/min lath martensite with some retained austenite takes over and hardness begins to fall. At 800 mm/min the structure is lamellar pearlite and blocky ferrite — transformation is starved of heat, and hardness collapses.
| Speed (mm/min) | Surface structure | Hardness / wear trend | Wear morphology |
|---|---|---|---|
| 100–200 | Blocky ferrite + pearlite | Low | Severe, oxide layer + cracking |
| 300 | Granular bainite + retained austenite | Rising | — |
| 400 | Fine acicular martensite | Peak hardness, min wear | Mild scratching |
| 600 | Lath martensite + retained austenite | Falling | Scratches, debris, ploughing |
| 800 | Lamellar pearlite + blocky ferrite | Low | Severe |
The wear evidence matches. At 100 mm/min the worn surface is severe, and EDS shows a high-oxygen oxide layer that softens and cracks under friction. At 600 mm/min the surface shows distinct scratches and loose debris — ploughing and abrasive wear act together. Only the 400 mm/min case wears lightly.
Power: 2 kW Is a Ceiling, Not a Floor
At 400 mm/min, power tells the same bell-curve story. At 1.5 kW the structure is flaky martensite; at 1.8 kW it is coarse lath martensite; at 2.0 kW it becomes fine acicular martensite with the best hardness and the lowest wear loss; at 2.2 kW it reverts to coarse lath martensite because the surface again crosses the melting threshold. The wear surfaces make the difference visible: at 2 kW the surface shows only slight scratches with fine adhered debris, while at 2.2 kW deep grooves and spall pits appear. The paper states it plainly: 2 kW produced a more wear-resistant hardened layer than 2.2 kW. More power is not more hardening — past the optimum it is overheating.
Buyers are taught to read laser-quench data as “higher hardness, better process”. The 24-run matrix flips that: hardness and wear resistance increase first and then decrease with both power and speed. The optimum sits inside a narrow golden window — 400 mm/min and 2 kW on a φ10 mm spot — where the surface peaks just below the 1600 °C melting threshold, the laser absorptivity is high, the self-cooling time is sufficient, and the structure becomes fine acicular martensite. Everything outside that window is either melted or untransformed.
What This Means for a Drill Tool Buyer
- Demand the process window, not a peak number: ask for the full parameter map (power × speed) and the microstructure evidence at the chosen setting.
- Treat “slower is safer” as a myth at the low end: below roughly 300 mm/min at this spot size, 4145H overheats and the phase transformation fails.
- Treat “more power is better” as a myth at the high end: 2.2 kW gave worse wear than 2.0 kW in the study.
- Verify with the temperature field, not just hardness: the simulated surface temperature of about 1550 °C at 2 kW / 400 mm/min, just below melting, is the physical signature of the optimum.
The Bottom Line
Laser quenching of 4145H drill tools is a window-finding exercise, not a power race. The 2021 study closes the loop that the 2019 paper opened: it extended the window to both failure modes and identified the golden combination — 2 kW, 400 mm/min, φ10 mm spot — that yields fine acicular martensite, peak hardness and the lowest wear. SHUNFU METAL supplies 4145H drill-collar stock and validates laser-quench windows with the same temperature-field logic before delivery, because on a drill string, the window is everything.
