When we talk about steel, we’re often referring to one of the most robust and versatile materials used in construction, manufacturing, and engineering. But what exactly makes steel so strong? A crucial factor is its tensile yield strength. Let’s dive into what this means and why it’s important.

### What is Tensile Yield Strength?

Tensile yield strength is a measure of how much stress a material can handle while being stretched before it starts to permanently deform. In simpler terms, it’s the maximum force per unit area that steel can withstand without changing shape. Imagine pulling on a rubber band: before it breaks, it stretches, and the point at which it starts to lose its original shape is similar to the yield strength.

#### Fundamentals of Yield Strength

**Yield strength** refers to the maximum stress a material can withstand before it starts to deform permanently. It marks the transition from elastic (temporary) deformation to plastic (permanent) deformation.

**Elastic Deformation**: The material returns to its original shape when stress is removed (e.g., stretching a rubber band).**Plastic Deformation**: The material retains a new shape after deformation (e.g., bending a paperclip).

On a stress-strain curve, yield strength is the point where the curve deviates from a straight line, indicating the start of permanent deformation. Understanding yield strength helps engineers select materials that can handle loads without permanent damage, ensuring structural integrity, safety, and effective performance.

## What is the Yield Strength of Steel?

The yield strength of steel varies significantly depending on its type and alloy. Typically, yield strength ranges from as low as 220 MPa (32,000 psi) for mild steel to as high as 1570 MPa (228,000 psi) for specialized high-strength alloys.

#### Yield Strength of A36 Steel

A36 steel, has a minimum yield strength of 250 MPa (36,000 psi). Its ultimate tensile strength ranges from 400 MPa to 550 MPa (58,000 psi to 80,000 psi).

#### Yield Strength of 304 Stainless Steels

The yield strength of 304 stainless steel is approximately 205 MPa (30,000 psi).

#### Yield Strength of 303 Stainless Steels

The yield strength of 303 stainless steel is approximately 415 MPa (60,200 psi).

#### Yield Strength of 316 Stainless Steels

Grade 316 stainless steel plate has a minimum yield strength of 207 MPa (30,000 psi) at 0.2% offset and a minimum tensile strength of 517 MPa7 (5,000 psi ).

#### Yield Strength of 17-4 Stainless Steels

17-4 stainless steel is known for its exceptional strength, with a yield strength ranging from 1,100 MPa to 1,300 MPa (160,000 psi to 190,000 psi).

#### Yield Strength of 1018 Steel

The yield strength of 1018 steel is approximately 370 MPa (53,700 psi).

#### Yield Strength of Grade 400 Steel

Grade 400 steel has a minimum yield strength of 420 MPa (61,000 psi) and a minimum ultimate tensile strength of 620 MPa (90,000 psi).

#### Yield Strength of 4140 Steel

4140 steel typically has a yield strength of 62,000 psi (427 MPa) and a tensile strength of 89,000 psi (614 MPa).

### How to Calculate Yield Strength of Steel?

To determine the yield strength of steel using a stress-strain diagram described by the equation ε = 0.20 × 10⁻⁶ σ + 0.20 × 10⁻¹² σ³, where σ is in kPa, and a 0.5% offset, follow these steps:

**Identify the Offset Strain**: For a 0.5% offset, the strain ε is 0.005.**Substitute ε into the Equation**: Set ε = 0.005 and solve for σ in the equation: 0.005 = 0.20 × 10⁻⁶ σ + 0.20 × 10⁻¹² σ³.**Solve for σ**: Rearranging and solving the cubic equation yields σ = 2810.078 kPa.

Thus, the yield strength of the steel is approximately 2810.078 kPa.

### How to Increase Yield Strength of Steel?

To increase the yield strength of medium manganese (Mn) steel, you can opt for processes that enhance the hard martensite matrix. Two effective methods are:

**Quenching and Partitioning (Q&P)**: This process involves rapidly cooling the steel to form martensite and then reheating to allow carbon to diffuse, resulting in a stronger steel matrix.**Combined Intercritical Annealing and Q&P**: This method first involves heating the steel to a temperature where both ferrite and austenite are present, followed by Q&P treatment to refine the microstructure and increase strength.

These techniques help achieve a significant boost in yield strength by optimizing the steel’s microstructure.

### How to Test Yield Strength of Steel?

One of the most common methods for testing the yield strength of steel is the tensile test. This method involves pulling a steel sample until it deforms, allowing for the measurement of its yield strength based on the stress-strain relationship. Although the tensile test provides accurate and reliable results, it is both destructive and time-consuming. As a result, it is typically used to assess the yield strength of materials from the same batch to ensure consistency.

## Factors Affecting Yield Strength

Several factors can influence the yield strength of steel:

**Alloying Elements**: Adding elements like carbon, chromium, and molybdenum can increase yield strength by altering the steel’s microstructure, making it more resistant to deformation.**Heat Treatment**: Processes such as quenching and tempering enhance yield strength by changing the steel’s grain structure, leading to improved mechanical properties.**Cold Working**: Deforming steel at room temperature through methods like rolling or forging increases its yield strength by introducing dislocations into the material’s structure, which makes it harder.

These factors work together to tailor the yield strength of steel for various applications and performance requirements.

### Yield Strength vs Tensile Yield Strength

Yield strength and tensile yield strength are often used interchangeably, but they can refer to slightly different concepts. Yield strength is the amount of stress a material can withstand before it begins to deform permanently, while tensile yield strength specifically refers to the yield strength measured during a tensile test.

**What Is the Symbol for Yield Strength?**

The symbol for yield strength is σY, where σ represents engineering stress and the subscript “Y” denotes “yield.” Additionally, the notation “SY” or “SX” can also be used to indicate yield strength in some contexts. This notation helps differentiate yield strength from other stress or strength measures in engineering.

### What Does Yield Strength 0.2% Mean?

Yield strength at 0.2% refers to the stress value at which a material begins to deform plastically, with a permanent strain of 0.2%. This metric is used to define the point at which a material transitions from elastic to plastic behavior, indicating the maximum stress it can withstand before undergoing irreversible deformation.

### How to Calculate Bolt Yield Strength?

For a Grade 5 bolt with a diameter of 12 mm and a yield load of 90,000 psi:

- Convert diameter to inches: 12 mm = 0.472 inches.
- Calculate the cross-sectional area:
**Area = π × (0.472/2)² ≈ 0.175 in²**. - Compute yield strength:
**Yield Strength = 90,000 psi / 0.175 in² ≈ 514,286 psi**.

## Conclusion

Yield strength is a crucial factor in determining the strength and durability of steel. By understanding yield strength, engineers and designers can make informed decisions about materials and design specifications to ensure safety and performance.

If you have any questions or need more information about yield strength or steel materials, feel free to ask!

**More resources:**

Steel Melting Point — Source: BOYI

Does White Steel Rust — Source: BOYI

## FAQ

**Is 0.2 proof stress the same as yield strength?**

0.2% proof stress is a specific measure of yield strength. It refers to the stress needed to cause 0.2% plastic deformation in a material. While it approximates yield strength, especially in materials without a clear yield point, it is not always identical.

**What is the relationship between proof strength and yield strength?**

While both proof strength and yield strength are indicators of a material’s ability to withstand stress, yield strength represents the maximum stress a material can handle before permanent deformation, and proof strength provides a practical measure used for engineering and safety purposes.

**What is an example of tensile strength?**

Tensile strength is a critical property that measures a material’s ability to withstand axial loads (pulling or stretching forces) without breaking. An excellent example of tensile strength in action is the use of **steel cables** in **suspension bridges**.

Catalog: Materials Guide

This article was written by engineers from the BOYI team. Fuquan Chen is a professional engineer and technical expert with 20 years of experience in rapid prototyping, mold manufacturing, and plastic injection molding.