Fushun Metal · Technical Notes
By Harris, technical staff at Fushun Metal
Sulfide Morphology, Not Sulfide Content: How 42CrMo1 Bar Actually Earns Its Machinability
The prevailing view is wrong for this grade. Almost every supplier page and textbook entry treats MnS as a defect to be driven toward zero. That is the right instinct for a bearing or a pipeline steel. It is the wrong instinct for 42CrMo1 destined for high-volume turning of automotive axle shafts and crankshafts. Here, sulfur is a deliberately added feature, and the entire melt shop sequence is engineered not to remove it but to reshape it.
Before the argument, let us place the grade on the world map, because buyers routinely ask us which Western number 42CrMo1 answers to. It is a Chinese GB chromium–molybdenum alloy structural steel sitting in the same family as the American AISI/SAE 4140 and the European EN 10083-3 42CrMo4, material number 1.7225. Chemically they overlap heavily; the practical difference is that the 42CrMo1 bar discussed here is resulfurized on purpose, holding sulfur at 0.015–0.025 % while a standard 4140 or 1.7225 heat would target sulfur as low as the furnace can take it.
Grade context: 42CrMo1, AISI 4140 and EN 1.7225 side by side
| Element | 42CrMo1 (GB) | AISI/SAE 4140 | EN 42CrMo4 / 1.7225 |
|---|---|---|---|
| C | 0.38–0.40 | 0.38–0.43 | 0.38–0.45 |
| Si | 0.20–0.30 | 0.15–0.30 | 0.15–0.40 |
| Mn | 0.74–0.80 | 0.75–1.00 | 0.65–1.00 |
| Cr | 1.12–1.18 | 0.80–1.10 | 0.90–1.20 |
| Mo | 0.16–0.20 | 0.15–0.25 | 0.15–0.30 |
| S | 0.015–0.025 | ≤0.035 (low) | ≤0.025 (low) |
Read the last row again. Where 4140 and 1.7225 push sulfur down, the 42CrMo1 bar specification holds it in a controlled band. That single decision reframes the whole metallurgical problem: the question is no longer “how little sulfide can we make” but “what shape will the sulfide take after rolling.”
The specification we are held to
The customer product is a 100–150 mm diameter hot-rolled round bar for axle and crankshaft service. Two tables define the target. The first is the chemistry window; the second is the non-metallic inclusion rating ceiling, graded against GB/T 10561.
| C | Si | Mn | P | S | Al | Mo | Cr |
|---|---|---|---|---|---|---|---|
| 0.38–0.40 | 0.20–0.30 | 0.74–0.80 | ≤0.02 | 0.015–0.025 | 0.01–0.03 | 0.16–0.20 | 1.12–1.18 |
| Type | A (sulfide) | B (aluminate) | C (silicate) | D (oxide) | Total |
|---|---|---|---|---|---|
| Thin series | ≤2.0 | ≤2.0 | ≤2.0 | ≤2.0 | ≤4.5 |
| Heavy series | ≤1.5 | ≤1.5 | ≤1.5 | ≤1.5 |
Why a long strip is worse than a fat spindle
During hot rolling, a ductile elongated MnS stringer simply flattens and stretches along the bar axis. It slices the steel’s continuity, drags toughness and fatigue down in the transverse direction, and produces the anisotropy that ruins a crankshaft. A globular or spindle-shaped inclusion does not stretch: it stays roughly equiaxed, chips cleanly, and acts as a beneficial break-chip point during machining. So the entire process below is aimed at one thing — converting long strips into oxide-cored spheres before the steel ever reaches the rolling mill.
Lever 1 — the dissolved-oxygen window at LF exit
This is the counter-intuitive part, and it is where most articles about “clean steel” go wrong for a free-machining grade. In the LF refining stage, the team does not chase the lowest possible oxygen. It holds a deliberate dissolved oxygen band of 0.0005–0.0010 % at tap-out. That trace oxygen lets MnS nucleate around oxides as a complex Mn,Fe,Ca,Al(S,O) inclusion rather than as a pure, plastic sulfide. A plastic sulfide deforms; a sulfide-oxide composite does not. Push oxygen to zero and you get beautiful cleanliness on paper and terrible machinability on the lathe. The only real cost of oxygen is above roughly 0.0010 %, where sub-surface gas defects and abrasive oxides begin to hurt the product.
Lever 2 — calcium treatment, then re-sulfurization
Calcium (Ca) treatment is the modification step. At LF exit, 50 m of pure calcium wire is fed for a micro-calcium dose, and after RH vacuum (held 10–15 min below 67 Pa) the calcium target is set at 0.0012–0.0015 %. Calcium does two jobs at once: it modifies stubborn Al₂O₃ clusters, and it seeds finely dispersed CaS and CaO·Al₂O₃ particles that form before MnS during solidification. Those particles become nucleation sites, so the sulfide that later precipitates wraps around a hard oxide core instead of freezing as grain-boundary films.
The timing is the craft. After an 8–10 min soft argon bubble, iron-sulfur cored wire is fed to raise sulfur back to 0.018–0.021 %. Sulfur is added after calcium, on purpose, so the re-sulfurization happens onto a steel already populated with oxide cores. The result at RH exit is a population of 10–20 µm spherical, CaS-nucleated calcium aluminate inclusions — exactly the “ball with a shell of (Ca,Mn)S” morphology the process was designed to produce.
Lever 3 — slag basicity, and Lever 4 — casting cooling
The slag basicity (CaO/SiO₂ ratio) is held at a deliberately moderate 3.0–5.0, not the extreme top-hung basicity a desulfurization purist would demand. Near the end of LF treatment, 150–250 kg of acidic silicon-sand fluxing agent is added to pull basicity down and stabilize the sulfur content — because for this grade you want the sulfur to stay put, not to be scrubbed into the slag. Final slag sits at CaO 50–55 %, MgO 4–7 %, Al₂O₃ 20–25 %, SiO₂ 12–15 % and FeO 1.0–1.5 %, with no aluminum adjustment in the last 20 min.
The fourth lever is at the caster. Because manganese and sulfur concentrate at the 1/2R position of a thick bloom, that is where coarse strip MnS is born. The fix is heat-extraction, not chemistry: mold water is raised from 3200 L/min to 3600 L/min and secondary cooling from 0.16 L/kg to 0.24 L/kg, with tundish superheat held to 20–30 °C under full argon-shielded casting. Faster solidification shrinks the time available for sulfide to segregate and grow, and samples taken at the 1/4-thickness of the billet show inclusions now dominated by spherical CaS-nucleated calcium aluminates ≤10 µm.
What the numbers prove
The optimized heats are not argued on philosophy; they are shown on the analysis sheets. Tundish chemistry landed inside the window on every heat, with sulfur held at 0.017–0.019 % and total oxygen at 0.0013–0.0016 % — the dissolved-oxygen band that keeps MnS globular.
| Heat | C | Si | Mn | P | S | Al | Cr | Mo | T.O |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 0.39 | 0.23 | 0.77 | 0.015 | 0.017 | 0.015 | 1.15 | 0.17 | 0.0016 |
| 2 | 0.39 | 0.22 | 0.75 | 0.012 | 0.017 | 0.016 | 1.14 | 0.16 | 0.0013 |
| 3 | 0.40 | 0.24 | 0.78 | 0.017 | 0.019 | 0.014 | 1.16 | 0.19 | 0.0015 |
After rolling to bar, the inclusion ratings confirm the payoff. Sulfide (A-type) sits at 0.5–1.5 grade, aluminates (B) and oxides (D) at 0.5–1.0, all inside the Table 2 ceilings, and no single heat exceeds 4.0 total against a 4.5 limit.
| Heat | A (sulfide) | B (aluminate) | C (silicate) | D (oxide) | Total | ||||
|---|---|---|---|---|---|---|---|---|---|
| Thin | Heavy | Thin | Heavy | Thin | Heavy | Thin | Heavy | ||
| 1 | 1.5 | 1.0 | 0 | 0.5 | 0 | 0 | 1.0 | 0 | 4.0 |
| 2 | 1.5 | 1.0 | 1.0 | 0 | 0 | 0 | 0.5 | 0 | 4.0 |
| 4 | 1.0 | 0.5 | 0 | 0 | 0 | 0 | 1.0 | 0 | 2.5 |
Micrographic evidence
Figure 1. Scanning-electron view of the sulfide at the 1/2R position of the bar (A-type, 1.5 grade, 100 µm scale). After optimization the sulfide is no longer a continuous stretched band; it appears as discrete, thickened particles with a much lower length-to-width ratio than the pre-treatment strip morphology.
Figure 2. Energy-dispersive spectrum of a single 1/2R inclusion, showing co-existing Mn, S, O, Al, Ca and Fe peaks. This confirms the target composite (Mn,Fe,Al,Ca)(S,O) chemistry — the oxide-cored sulfide that resists deformation in rolling and is the physical reason machinability improves.
The takeaway for a buyer
When you specify 42CrMo1, AISI 4140 or EN 1.7225 for machined axle and crankshaft parts, the certificate line for sulfur tells you almost nothing about how the bar will behave on your machine. Two heats at the same 0.02 % sulfur can differ by hours of tool life, because one froze as long strip MnS and the other as oxide-cored spheres. What actually governs performance is the melt-shop discipline behind it: a controlled dissolved-oxygen window, calcium dosed before re-sulfurization, a moderate 3.0–5.0 basicity that keeps sulfur in solution, and an aggressively cooled bloom.
Ask your supplier not “how low is your sulfur” but “what shape is your sulfide, and can you show me the 1/2R rating.” That single reframe separates a bar that machines cleanly from one that quietly costs you a crankshaft.
