The 4 key factors in choosing the right steel: strength, hardness, toughness, and Weldability

Steel is everywhere—in mechanical manufacturing, construction, the automotive industry, and even in our daily lives. However, when faced with the wide variety of steel types on the market, many people feel confused.Because steel is not a single concept, its properties vary greatly. Choosing the wrong steel can, at best, lead to product failure and wasted costs, and at worst, cause serious safety accidents.

Actually, we don’t need to be metallurgy experts to make wise choices.Understanding the following four key properties—strength, hardness, toughness, and weldability—is like mastering the ‘four essential tools’ for selecting steel.

Key Point 1: Strength—Resistance to Deformation and Fracture

Strength is the most commonly mentioned property of steel. It measures the material’s ability to resist permanent deformation and fracture. We can think of it as a person’s “muscle power.” When an external force is applied to the steel, internal stress develops in resistance. Strength is the maximum stress the steel can withstand before yielding or breaking.

Main indicators of strength:

(1) Yield strength: This is the most critical strength indicator. It refers to the stress value at which steel begins to exhibit noticeable plastic deformation (i.e., deformation that cannot return to its original shape). When designing structural components, yield strength serves as the basis for calculations, and it is essential to ensure that the working stress is well below the yield strength to guarantee that the structure does not undergo permanent deformation under normal use.

(2)Tensile strength: refers to the maximum stress that steel can withstand before breaking. It represents the ultimate load-bearing capacity of the material. Although structural design usually does not rely on tensile strength, the ratio of tensile strength to yield strength (strength ratio) is an important parameter for assessing material safety margins. The smaller the strength ratio, the more obvious the plastic deformation of the structure before failure, and the higher the safety.

In daily life, for example, the boom of a crane must have extremely high strength to lift several tons of weight without stretching or bending; the strength of the steel structure in a high-rise building determines how tall the building can be; the load-bearing axles of a car also need high strength to support the weight of the car body and cargo.

So, how do we determine it?

First, look at the use: if the materials you purchase are labeled ‘high-strength steel’ or a specific grade (such as Q345B, where ‘345’ roughly indicates the yield strength), it usually means it is suitable for load-bearing structures.

Secondly, you can do a simple test: for small parts, try applying force with your hand or a simple tool. If it bends easily and doesn’t spring back, it indicates low strength. However, this is just a very rough assessment. This method should be done with caution.

Finally, let’s not fall into the trap of choice; blindly pursuing high strength is not wise. High-strength steel often comes with a decline in other properties (such as toughness and weldability), as well as higher cost and processing difficulty. For a bracket that only needs to bear light loads, using ordinary Q235 steel may be more economical and practical than using high-strength alloy steel.

Summary: When your design needs to withstand huge weight or pressure, and deformation is unacceptable, you should prioritize high-strength steel.

Key Point Two: Hardness – Resistance to Surface Penetration
Hardness measures the steel’s ability to resist being indented or scratched by harder objects.

Common methods for testing hardness:

(1) Brinell Hardness: Using a hard ball to press into the material’s surface, the hardness value is calculated by measuring the diameter of the indentation. Suitable for materials with lower hardness and coarser grains.

(2)Rockwell Hardness: Uses a diamond cone or steel ball indenter to indicate hardness based on the depth of the indentation. It is easy to operate, highly efficient, and the most widely used.

(3)Vickers Hardness:Uses a diamond-shaped square pyramid indenter to measure the diagonal length of the indentation. It is highly precise and suitable for hardness testing of thin layers and small areas.

The application scenarios are as follows:

(1) Gears and bearings: These components come into contact with each other during high-speed operation, bearing immense contact stress and friction. High hardness ensures they have excellent wear resistance and extends their service life.

(2)Excavator bucket teeth and crusher jaw plates: they directly collide and rub against hard materials such as rocks and ores, and must have extremely high hardness to withstand severe abrasive wear.

(3)Cutting tools and molds:Cutting and forming tools must ensure that the hardness of the cutting edge or cavity is much higher than that of the material being processed in order to maintain sharpness and shape.

Many people think that strength and hardness are the same, but that is not entirely correct. For example, a piece of glass is very hard and cannot be scratched with a key, but it is very brittle and will shatter if dropped; a high-strength spring steel can be bent to a large angle without breaking, but its surface may be easily scratched by hard objects. On the other hand, the harder a material is, the more brittle it usually becomes. A chisel with extremely high hardness may break under impact rather than deform plastically. Therefore, for parts that are subjected to impact, a balance between hardness and toughness needs to be found.

Summary: When your design needs to resist friction and wear or requires maintaining sharpness, prioritize high-hardness steel.

Key Three: Resilience — Absorbing the ‘Impact’

Toughness is the ability of a material to absorb energy and undergo plastic deformation before fracturing. It describes the ‘flexible’ aspect of a material and represents a combination of strength and ductility.Simply put, toughness measures a steel’s ability to absorb energy and resist fracturing when subjected to impact or sudden force. If strength is about resisting ‘slow bending,’ then toughness is about resisting being ‘smashed all at once.’ A tough material will deform and bend before breaking, giving you a ‘warning.’ A material without toughness is ‘brittle’ and will fracture suddenly, like glass.

Another very important concept is the ductile-to-brittle transition temperature. Many steels will transition from a ductile state to a brittle state at low temperatures. Above this temperature, the steel exhibits noticeable plastic deformation before fracture, and the fracture surface appears fibrous.When the temperature is below this point, steel almost suddenly fractures without significant plastic deformation, and the fractured surface appears crystalline, which is extremely dangerous. Many historical incidents of ships and bridges experiencing brittle fractures in severe cold were caused by the steel’s ductile-to-brittle transition temperature being higher than the ambient temperature.

Core measure of toughness: impact toughness

It is usually measured by the Charpy impact test. A notched standard specimen is placed on an impact testing machine and broken in a single strike by a pendulum hammer. The work consumed to fracture the specimen is measured, which is called the impact energy. The higher this value, the better the material’s toughness.

The application scenarios are as follows:

(1) Automotive crash beams: In the event of a collision, they need to absorb impact energy through their own bending deformation to protect passengers. They must be tough, not brittle.

(2)Hammerhead of the hammer: When driving nails, it bears a huge impact. If it lacks toughness, it may crack and send off fragments, which is very dangerous.

(3)Bridges: They need to withstand strong winds, earthquakes, and dynamic loads from vehicle traffic. Toughness ensures they do not suddenly collapse under vibrations.

Summary: When your design needs to withstand impact and vibration, or when a clear warning is required before breaking, prioritize high-toughness steel.

Key Point Four: Weldability — Achieving a ‘Firm Connection’

Weldability refers to the ease with which steel can form high-quality weld joints under certain welding process conditions. Simply put, weldability measures whether steel can be welded together easily and reliably, and whether the performance of the welded joint (strength, toughness) is close to that of the base material.

Key factors affecting weldability: carbon equivalent

The carbon and other alloying elements (such as manganese, chromium, vanadium, etc.) in steel can significantly affect its welding performance. To evaluate this comprehensively, the concept of “carbon equivalent” is introduced. The formula converts the effects of various alloying elements on hardenability into an equivalent carbon content. The higher the carbon equivalent, the greater the steel’s tendency to harden, and the poorer its weldability.

Risk of high carbon equivalent: During the rapid heating and cooling process of welding, the weld and heat-affected zone are prone to forming hard and brittle martensitic structures, leading to cold cracking. These cracks are very subtle and may propagate during use, causing sudden structural failure.

Application Scenarios:

(1) Steel structure factories and wrought iron gates: Almost entirely made of low-carbon steel with excellent welding performance.

(2)Car chassis and frames: Made of high-strength low-carbon steel or special steel, ensuring both strength and meeting the process requirements for large-scale robotic welding.

(3) A traditional forged samurai sword: It is made of high-carbon steel with excellent performance, but you can hardly see any welds on it because it is not designed for welding in the first place.

Summary: If your project involves welding, make weldability one of the top considerations, which usually means choosing low-carbon steel.