Understanding Temper Grades in Copper Alloy Bearings: What Do They Really Mean?

When selecting copper alloy bearings, engineers often encounter the term “temper grade” or “annealing grade.” At first glance, it might seem like a single parameter, such as hardness. However, the reality is far more nuanced.

In this article, we break down the true meaning of temper grades, why they matter, and how they influence material selection for specific operating conditions.

What Is a Temper Grade?

The temper grade of a copper alloy bearing represents the result of a precisely controlled heat treatment process (heating, holding, and cooling). This process modifies the material’s microstructure to achieve a balanced combination of:

• Hardness

• Strength

• Plasticity

• Wear resistance

• Fatigue resistance

In short, the temper grade is not a single property but a “performance package”, indicated by its grade number.

Why Heat Treatment (Annealing or Precipitation Hardening) Is Necessary

Most copper bearing alloys, such as tin bronze (ZCuSn5Pb5Zn5, ZCuSn6Pb6Zn3) or lead bronze, do not exhibit optimal properties in their as-cast or sintered state. Their microstructure may be inhomogeneous, and residual stresses can lead to distortion or cracking.

The heat treatment process serves several purposes:

1. Stress Relief – Reduces internal stresses to prevent deformation during machining or in service.

2. Microstructure Homogenization – Ensures alloying elements (Sn, Pb) are evenly distributed in the copper matrix.

3. Control of Second Phase Precipitation – In tin bronze, controlling the δ-phase (Cu₃₁Sn₈) is critical. Its size and distribution determine whether the material is hard and strong or soft and ductile.

4. Optimized Hardness-Toughness Balance – The primary goal of the temper grade system.

Temper Grade Examples: Grade 1 vs. Grade 2

The two most common temper grades in industry are Grade 1 (hard) and Grade 2 (soft). While definitions may vary slightly by manufacturer, the following table illustrates the industry consensus:

Choosing the Right Temper Grade: Application-Centered Logic

Understanding temper grades is meaningless without considering actual operating conditions. Selection depends on:

Analogy:

• Grade 1 = Armor – hard, wear-resistant, but less forgiving.

• Grade 2 = Sponge – flexible, absorbs shocks, protects the shaft, but less wear-resistant.

Practical Implications for Bearing Design and Selection

1. Performance Labels, Not Just Numbers – Temper grade communicates the complete mechanical profile, not just hardness.

2. Microstructural Basis – The δ-phase distribution is the microscopic mechanism behind softening or hardening.

3. Operational Fit – Selecting the wrong temper grade can lead to premature wear, fatigue failure, or misalignment issues.

⚠️ Note: Always confirm both the alloy designation and the temper grade during specification. Ignoring temper grades can compromise the bearing’s life and performance.

Summary

• Temper grade = performance package: hardness, strength, plasticity, wear resistance, and fatigue resistance combined.

• Grade 1 (hard) is for high load, low speed, well-lubricated scenarios.

• Grade 2 (soft) is for lower load, high speed, vibration-prone, or contaminated environments.

• Microstructure control, especially of the δ-phase, is key to achieving the desired mechanical properties.

Selecting the correct temper grade ensures optimal bearing performance, longevity, and reliability, rather than simply following material labels.

Understanding Copper Alloy Temper Designations

The mechanical properties of copper and its alloys are significantly altered by the thermomechanical treatments they undergo during or after production. A standardized temper designation system is used for all wrought and cast copper alloy products (except ingots) to define their final material condition or state. This system is based on the sequence of fundamental processing operations that produce these various tempers.

The temper designation follows the alloy number, separated by a hyphen. The basic designation consists of a letter, which may be followed by one or more digits to represent subdivisions of that temper. These numbers specify the sequence of operations but only include those that significantly influence the product’s final characteristics.

The following tables outline the temper classifications and terminology commonly used by copper and copper alloy manufacturers in the United States.

Annealed Tempers (O Series)

These tempers are achieved through annealing processes to control either grain size or specific mechanical properties.

OS Series: Annealed to a Specific Grain Size

The material is annealed to meet the requirements of a specified average grain size.

O Series: Annealed to Specific Mechanical Properties

The material is annealed to meet specific mechanical property requirements.

Cold-Worked Tempers (H Series)

These tempers are achieved by cold working, often followed by stress-relieving or other thermal treatments.

As-Fabricated Tempers (M Series)

These designations describe the material’s condition immediately after casting or hot working processes without subsequent thermal treatment.

Heat-Treated Tempers (T Series)

These tempers are achieved through solution heat treating, quenching, and precipitation (age) hardening processes.

Combination Tempers (TH, TM, TS, TL Series)

These tempers are achieved through a combination of cold working and heat treatment.

Welded Tube Tempers (W Series)

These designations are specifically for welded tubing to indicate its condition after forming, welding, and subsequent processing.