UNS C90300 Tin Bronze — Properties, Composition & Casting Guide

Introduction

UNS C90300 tin bronze is the go-to casting copper alloy when you need wear resistance, corrosion durability, and reliable castability in one material. Engineers across marine, pump, and bearing applications have specified C90300 for decades — because it consistently delivers where balanced performance matters more than peak strength.

This guide breaks down exact composition ranges, as-cast mechanical values, foundry process parameters, and the cost logic that determines when C90300 beats its closest rivals. By the end, you’ll know precisely when to specify it, when to walk away, and which casting process to call out on your drawing.

C90300 vs C93200 vs C95400 — Quick Comparison

Picking the wrong alloy costs you in rework, premature wear, or over-engineered procurement. Here’s how C90300 stacks up against the two alloys it most commonly competes with.

C90300 wins when you need both corrosion resistance and bearing performance in the same part. C93200 underperforms in seawater. C95400 costs more and machines harder than most bearing applications justify.

UNS C90300 Composition — Alloy Chemistry Breakdown

C90300 is a copper-tin-zinc casting alloy. Its strength comes directly from tin content — not from heat treatment.

Always request the mill certificate for your exact heat. Tin sitting at 7.5% vs 9.0% produces measurable differences in hardness and wear life.

One spec detail professionals miss: C90300 is not a leaded bronze. Its trace Pb level (≤0.25%) keeps it compliant with most RoHS and potable water restrictions — something C93200 cannot claim.

UNS C90300 Mechanical Properties — As-Cast Values

These values reflect sand-cast and investment-cast production parts. Section thickness and solidification rate move these numbers — thicker sections cool slower and produce slightly lower strength.

Mechanical Properties

Physical Properties

The 18–30% elongation range is wide by design. Investment casting produces the higher end; sand casting of heavy sections lands lower.

Specify your casting process and wall thickness when requesting certified properties from your foundry.

C90300 Casting Process — Sand, Investment & Shell Mold Guide

C90300 runs well across three primary casting processes. Your geometry and tolerance band determine the right choice — not alloy preference.

Sand Casting suits large, heavy-section parts like valve bodies and pump housings. It’s the most economical route and works well where ±0.5 mm tolerances are acceptable.

Use ceramic filters in the gating system to reduce oxide inclusions. Tin bronzes are sensitive to hydrogen porosity when melt cleanliness slips.

Investment Casting (Lost-Wax) delivers the tightest tolerances (±0.1–0.2 mm) and best as-cast surface finish (Ra 1.6–3.2 µm). Use it for bushings with precision bore requirements, intricate pump impellers, or marine fittings with complex geometry.

The process costs 30–60% more per kilogram than sand casting. It eliminates most post-cast machining on critical surfaces — so total part cost often stays competitive.

Shell Mold Casting sits between the two in surface finish and tooling cost. It’s the right call for medium-complexity parts in production volumes above ~500 pieces.

Critical foundry parameters to call out on your drawing:

Pouring temperature should stay within the 1000–1050 °C window. Excessive superheat drives oxidation and dross formation.

Rotary degassing or inert gas purging is non-negotiable. Hydrogen porosity in tin bronze reads as smooth, rounded voids on X-ray — demand radiographic testing for any pressure-containing parts.

Specify chills on heavy sections to promote directional solidification. Include this in your casting design review, not as a post-processing correction.

Post-cast stress relief at 260–290 °C for 1–2 hours removes residual casting stress without altering mechanical properties.

C90300 does not respond to precipitation hardening. Any supplier offering “heat treated C90300 for higher strength” is either misidentifying the alloy or referring to stress relief only.

C90300 Machinability, Joining & Surface Finishing

Machinability is moderate — better than aluminum bronze (C95400), comparable to most unleaded tin bronzes. Use carbide tooling with coated grades for production runs.

Target surface speeds of 150–250 m/min for turning operations. Use light finishing passes on bearing bores to achieve Ra ≤ 1.6 µm for lubricated contact surfaces.

Built-up edge is the common failure mode in production. Rigid fixturing and consistent cutting fluid application solve it in most setups.

Joining by brazing is the accepted method for C90300 assemblies. Use a phosphor-copper or silver-copper filler with compatible flux rated for bronze.

Fusion welding introduces heat-affected zone cracking risk. Reserve it for non-structural repairs only, followed by stress relief.

Surface finishing options depend on function. Bearing surfaces require honing or lapping to Ra 0.4–0.8 µm for reliable lubricant film formation.

Decorative parts accept nickel or silver plating well after mechanical polishing. For seawater-exposed hardware, lacquer or patination finish reduces early-stage staining without affecting corrosion performance.

C90300 Corrosion Resistance — Marine & Seawater Performance

C90300 performs reliably in both fresh and seawater environments. Tin stabilizes a protective surface film that resists general corrosion, biofouling, and the dezincification that disqualifies many brasses from marine service.

In continuous seawater immersion, C90300 typically shows corrosion rates below 0.05 mm/year. That’s acceptable for multi-year service intervals on pump and valve parts.

Where C90300 shows limits: high-velocity seawater flow above ~2–3 m/s can trigger erosion-corrosion, accelerating surface loss faster than static corrosion data predicts. In those conditions, step up to C95400 aluminum bronze.

Galvanic compatibility matters in mixed-metal assemblies. C90300 is relatively noble — pairing it with aluminum alloys or carbon steel without isolation creates a galvanic cell. Specify insulating bushings or coatings at dissimilar metal interfaces in marine installations.

C90300 Cost & Decision Logic — When It Makes Economic Sense

Tin is a commodity metal with price volatility. C90300’s 7.5–9.0 wt% tin content directly affects material cost — that’s the first lever to check against your application requirements.

C90300 makes economic sense when all three of these are true:

The part requires corrosion resistance in wet or marine environments — ruling out C93200, which corrodes rapidly in seawater.

Load and speed stay moderate (PV value under ~1.75 MPa·m/s for unlubricated contact) — below the threshold where C95400’s higher strength becomes necessary.

Section geometry is castable in sand or shell mold. If investment casting is already required for tolerance reasons, C90300’s moderate premium over C93200 disappears in total part cost.

When to move to C93200 instead: high-volume, economy bushings running in oil-lubricated gearboxes with zero seawater exposure. C93200 machines faster, costs less per kilogram, and performs identically in dry or lubricated sliding contact.

When to move to C95400 instead: propeller hubs, heavy-duty marine gear bushings, or components exposed to both high mechanical load and high-velocity seawater. C95400’s UTS reaches 650 MPa vs C90300’s 320 MPa — that gap justifies the higher cost for safety-critical parts.

Quick decision rule: wet service at moderate load — specify C90300. Dry, high-volume, economy — specify C93200. Wet and high-stress — specify C95400.

UNS C90300 Applications — Bushings, Pumps, Marine & Gears

C90300 covers a consistent set of application categories across industries. Each one exploits a different combination of the alloy’s core properties.

Bushings and Plain Bearings — the most common application. C90300’s conformability and embedability allow it to absorb small contaminant particles into the softer bronze matrix, protecting the hardened steel shaft. Specify it for hydraulic pump bushings, gearbox sleeve bearings, and pivot bushings in moderate-load linkages.

Pump and Valve Components — impellers, valve bodies, and seats in water-handling and marine equipment. The alloy’s castability allows complex internal geometry in a single casting, reducing assembly cost. Corrosion resistance eliminates the coating maintenance cycle required for ferrous alternatives.

Marine Fittings and Hardware — seacocks, through-hull fittings, and deck hardware in continuous seawater exposure. C90300 meets the corrosion performance requirement without the cost premium of C95400 for non-structural fittings.

Worm Wheels and Small Gears — C90300’s ~70 HB hardness and good sliding wear resistance make it a practical pairing against hardened steel worm shafts. The softer bronze sacrifices itself preferentially, protecting the steel — the economically correct trade-off.

Architectural and Decorative Castings — where patina, aesthetics, and castability matter more than mechanical performance. C90300’s lead-free composition is also an advantage for interior applications with environmental material restrictions.

Conclusion on UNS C90300 Tin Bronze

C90300 has stayed in continuous production specification for one reason — it solves the right problem at the right price point. It’s not the strongest bronze, not the easiest to machine, and not the cheapest.

It’s the one that handles moderate load, resists corrosion reliably, casts cleanly across multiple processes, and stays compliant with environmental regulations that are increasingly eliminating leaded alternatives. When your application sits at the intersection of wear performance and wet-environment durability, C90300 is the alloy you specify with confidence — not the one you settle for.

UNS C90300 FAQs — Properties, Casting & Material Selection

What is the typical tin content in C90300?

Tin runs at 7.5–9.0 wt%, with copper making up the balance at 86–89%. Zinc adds another 3–5% to improve melt fluidity. Always verify against the foundry’s mill certificate for your specific heat.

Is UNS C90300 lead-free?

Effectively yes — lead is a trace impurity at ≤0.25 wt%, not an intentional alloying element. This keeps C90300 compliant with most RoHS directives and potable water regulations. It is not classified as a free-machining leaded bronze.

Can C90300 be heat treated to increase strength?

No. C90300 is not a precipitation-hardening alloy. Strength is controlled entirely by composition and solidification rate. Stress-relief anneals at 260–290 °C improve dimensional stability but do not raise mechanical properties.

Is C90300 suitable for seawater service?

Yes. Corrosion rates in seawater typically stay below 0.05 mm/year under static or low-velocity conditions. High-velocity erosion-corrosion scenarios above ~2–3 m/s require a step up to C95400.

What casting process should I specify for C90300?

Use investment casting for tight tolerances (±0.1–0.2 mm) and complex geometry. Use sand casting for large, economical parts where ±0.5 mm is acceptable. Shell mold casting covers medium-complexity, medium-volume production.

What is C90300’s machinability rating?

C90300 falls in a moderate machinability class — better than aluminum bronze, comparable to other unleaded tin bronzes. Use carbide tooling at 150–250 m/min for production turning. It machines significantly harder than leaded C93200.

C90300 vs C93200 — which is better for seawater bearings?

C90300. C93200’s lead content makes it vulnerable to corrosion in seawater and disqualifies it for marine wet service. C90300’s tin-driven corrosion resistance is specifically why it dominates marine bushing applications.

What pouring temperature should the foundry use for C90300?

Target a narrow band above the liquidus (~1000 °C). Most foundries pour at 1020–1060 °C depending on section complexity. Excessive superheat increases oxidation and dross — demand the foundry’s pour temperature records as part of your quality documentation package.