Transmission Steel

Plain Carbon Steels (Manufacturing Steels): Definition & Scope

Plain carbon steels—also called unalloyed machine-building steels or manufacturing steels—are non-alloy steels with approximately 0.20–0.60% C. As carbon increases, hardenability and attainable surface hardness rise, while toughness and weldability generally decrease. For thicker sections or where higher through-hardening is required, low-alloy steels are typically preferred.

Metallurgy & Key Properties

Plain carbon steels provide moderate strength, predictable behavior in surface hardening, and a cost-efficient balance of properties for general mechanical parts.

• Hardenability vs. carbon: Higher carbon improves response to quench/induction hardening; however, thick sections may not fully harden through the core.

• Strength–toughness trade-off: Strength and hardness increase with carbon, while toughness and ductilitydecline.

• Microstructure control: Through hot rolling, normalizing, quenching and tempering, or surface hardening, the matrix (ferrite/pearlite/martensite) is tuned to need.

Processing Routes

Cold Drawing (Room-Temperature Deformation)

Bars are pulled through dies to refine dimensions, straightness, and surface finish while raising yield and tensile strength without a full hardening cycle. After heavy reductions, stress-relief may be applied.

Cold Rolling (Thickness Reduction)

Used primarily for sheet/strip where property improvement is achieved by thickness reduction; products are referred to as cold-rolled steels.

Surface Hardening Options

• Carburizing (≤0.25% C): Builds a hard, wear-resistant case over a tough core—common for gears, pins, and wear surfaces.

• Flame/Induction hardening (≥0.40% C): Directly hardens the surface of medium-carbon grades (e.g., C45) for high wear resistance with retained core toughness.

Machinability & Weldability

• Machinability: Lower than low-carbon steels but acceptable; resulfurized variants enhance chip control and tool life.

• Weldability: Declines with carbon content. Use preheat/interpass control and proper procedures on medium-carbon grades to avoid HAZ cracking and preserve toughness.

Applications

Designed for machine elements not requiring high alloy strength, including shafts, pins, gears and gear components, bolts, axles, hooks, mold sets, and general hardware in vehicle, engine, machine, and equipmentmanufacturing. The machine-building sector relies heavily on these steels for robust, economical parts.

Grades & Designations (Examples)

Common descriptors include SAE 1030/1040/1050/1060 and their European counterparts C30/C40/C50/C60. Resulfurized versions (e.g., C30R/C40R/C50R/C60R) improve machinability; typical S content is 0.020–0.035%.

When to Choose Low-Alloy Alternatives

If your design demands higher hardenability, deeper case depth on thick sections, elevated toughness, or better strength-to-weight, step up to low-alloy grades (e.g., Cr-Mo, Ni-Cr-Mo systems) with appropriate heat treatment.

Plain Round Steel Bars (Smooth Round) — Advanced Overview

Plain round steel bars—often called smooth round bars or simply rounds—are the most common long-product form for manufacturing steels. They are the workhorse input for rotational and load-bearing components where consistency, finish, and dimensional control directly impact performance.

What They Are

Cylindrical bars supplied as hot-rolled, cold-drawn (bright), peeled/turned, or ground products. The chosen route affects tolerance class (e.g., h9/h11), straightness, roundness, and surface roughness—and ultimately bearing life, seal wear, and assembly precision.

Production Routes & Finishes

• Hot-Rolled Rounds: Economical for larger diameters and rough machining. Scale present; tolerances are looser but suitable where parts will be turned/grinded later.

• Cold-Drawn (Bright) Rounds: Enhanced strength, straightness, and surface quality with tighter h9/h11tolerances—ideal for general shafts and precision fit components.

• Peeled/Turned Rounds: Scale removed and diameter trued; an excellent pre-grind condition with stable machinability.

• Ground/Polished Rounds: Tightest tolerances and lowest roughness for bearing journals, seal lands, and high-speed shafts.

Typical Sizes & Tolerances (Indicative)

• Diameters: Commonly Ø5–Ø100 mm (larger on request).

• Lengths: 6 m and 12 m standard; cut-to-length service reduces scrap and setup time.

• Tolerances: h11 for general purpose; h9 (or tighter if ground) for bearing fits and precision assemblies.

Performance Advantages

• Dimensional stability & straightness reduce vibration, misalignment, and premature bearing wear.

• Improved surface integrity enhances fatigue resistance and running accuracy.

• Lower total machining time with bright/peeled/ground stock; better chip control on cold-drawn material.

• Predictable case-hardening and induction response on suitable carbon levels (e.g., C30–C60, C45).

Where They’re Used

• Power transmission: drive and intermediate shafts, pins, spindles, hubs.

• General mechanical: gears/gear hubs (with case hardening), couplings, threaded fasteners, bush carriers.

• Automotive & industrial equipment: axles, steering/column parts, conveyor and crane shafts, fixtures and tooling.

Ordering Checklist (What to Specify)

• Grade & condition: SAE 1030/1040/1050/1060, C30/C40/C50/C60; resulfurized “R” options if machinability is critical.

• Diameter, length & tolerance class: h11 for general; h9 or ground for precision fits.

• Finish: hot-rolled, cold-drawn (bright), peeled/turned, ground/polished.

• Heat treatment/surface hardening: as-rolled/normalized, Q&T, carburized, or induction hardened.

• Quality documentation: request EN 10204 3.1 MTC, straightness/roundness criteria, and any additional NDT requirements.

Practical Selection Notes

• Use carburizing on low-carbon rounds (<0.25% C) when you need a hard, wear-resistant case with a tough core (e.g., small gears, pins).

• Choose flame/induction hardening on medium-carbon rounds (≈0.40–0.55% C, e.g., C45/C50) for localized surface hardness with minimal distortion.

• For thick sections or high duty cycles, consider low-alloy rounds to achieve deeper hardening and better fatigue toughness.