Strength vs. Toughness vs. Hardness: Understanding the Trifecta of Steel Performance

Property 1 of Steel: Strength

(1) What is Steel Strength?

Strength is the core ability of steel to resist damage—specifically fracture (like a steel bar snapping) or permanent deformation (like a bent steel pipe that can’t straighten back)—when it is subjected to different types of stress, such as pulling, pressing, or twisting.

(2) Main types and characteristics of Steel Strength

1. Tensile strength: It is the ability to resist stretching. For example, construction steel bars need to bear their own weight and wind force. What standard does high-strength threaded steel follow? The ASTM A615 Standard requires its tensile strength to be no less than 550MPa.
2. Compressive strength: It is the ability to resist compression. For example, mechanical bases need to bear weight stably. What rule does carbon steel for mechanical bases follow? The ISO 8373 Standard stipulates its compressive strength to be no less than 250MPa.
3. Yield strength: It is the stress when plastic deformation starts. For example, machine tool guide rails need to avoid deformation. What requirement does S355 steel meet? In the EN 10025-2 Standard, its yield strength is no less than 355MPa.

(3) Factors that affect Steel Strength

1. Chemical composition: Higher carbon content brings higher strength. How to improve tensile strength? Adding 0.8%-1.2% manganese can increase tensile strength by 20%-30% (based on ASM Metals Handbook).
2. Microstructure: Martensite has the highest strength. How does the tensile strength of Grade 45 steel change? After quenching and tempering, its tensile strength rises from 600MPa to over 1000MPa (based on JIS data).
3. Processing technology: Cold-rolled steel plates have higher yield strength than hot-rolled ones. What is the specific difference? Their yield strength is 30%-50% higher (based on World Steel 2023 Report). Where are they used? They are suitable for car bodies.

(4) Testing method for Steel Strength

People mainly measure steel strength through the “tensile test”. This method can accurately obtain two key indicators: yield strength and tensile strength. It follows the ISO 6892-1 Standard throughout to ensure reliable results.

When conducting the test, you first process the steel into a standard sample. The sample is usually a long strip with a thinner diameter in the middle. You also mark two “gage marks” on the thin middle part (the distance between them is mostly 50mm).

Next, you clamp both ends of the sample into a tensile testing machine. The machine applies pulling force to both ends at a slow and stable speed (2-5mm per minute). At the same time, it records the force magnitude and the elongation between the gage marks.

As the pulling force increases, the sample first undergoes elastic deformation (it can recover after the force is removed). When the pulling force reaches the “yield strength”, the sample starts to show permanent deformation— the distance between the gage marks no longer shortens when the force stops.

You continue to apply more force until the sample breaks at the thin middle part. The maximum force at this moment is the “tensile strength”.

This test not only helps judge whether steel meets standards (for example, construction steel must meet the 550MPa tensile strength requirement in the ASTM A615 Standard), but also provides a basis for selecting steel for different scenarios. For instance, steel used in car bodies needs high yield strength to avoid excessive deformation during collisions.

Property 2 of Steel: Toughness

(1) What is Steel Toughness?

Toughness is the key ability of steel to absorb energy—like the energy from a collision, a drop, or a sudden hit—and resist breaking (fracture) even when facing those forces.

(2)Why is Steel Toughness important?

Insufficient toughness easily causes accidents in low-temperature or impact scenarios. For example, what standard do oil pipelines in cold areas need to meet? The API 5L Standard requires their Charpy impact energy at -40℃ to be no less than 40J.

(3)Factors that affect Steel Toughness

1. Temperature: Low temperature reduces toughness. What is the brittle transition temperature of 9% nickel steel? It is as low as -196℃ (based on DNV data). Where is it used? It is suitable for LNG storage tanks.
2. Chemical composition: Phosphorus and sulfur reduce toughness. How to improve low-temperature toughness? Adding nickel with a content of no less than 3.5% can achieve this (based on BS EN 10028-4 Standard).
3. Microstructure: Grain refinement improves toughness. How does impact toughness change? When grains shrink from 100μm to 10μm, impact toughness increases by 2-3 times (based on VDEh Report).

(4) Testing method for Steel Toughness

People often test steel toughness through the “Charpy impact test”. Its core goal is to measure steel’s ability to absorb impact energy. The operation follows the ASTM A370 Standard, and it can also help determine steel’s “brittle transition temperature”.

Before the test, you first cut the steel into a standard sample (10mm thick, 10mm wide, 55mm long). You then cut a V-shaped or U-shaped notch in the middle of the sample (the notch concentrates the impact force and makes results more accurate).

Next, you place the sample on the bracket of an impact testing machine, making sure the notch faces away from the impact direction. You then lift a pendulum of specified weight to a fixed height— the height determines the pendulum’s initial energy.

After releasing the pendulum, it falls quickly and hits hard above the sample’s notch. The sample either bends or breaks. The machine automatically calculates the energy lost by the pendulum after the impact. This energy is called “impact energy” (unit: joule, J). The higher the impact energy, the better the steel toughness.

By conducting this test at different temperatures, you can find the critical temperature at which steel changes from tough to brittle (for example, low-temperature pipeline steel needs to have an impact energy of ≥40J at -40℃, which meets the API 5L Standard). This ensures steel will not break suddenly in cold environments.

Property 3 of Steel: Hardness

(1) What is Steel Hardness?

Hardness is the ability of steel surface to resist indentation and wear. It can indirectly infer strength.

(2)Main types and applications of Steel Hardness

1. Brinell Hardness (HB): It tests medium and low hardness steel, such as machine tool castings. What test conditions does it follow? The ISO 6506-1 Standard specifies a 10mm steel ball and 3000kgf pressure for testing.
2. Rockwell Hardness (HR): It tests steel of different hardness levels. What scale is used for cutting tools? HRC scale. What standard is widely adopted? The ASTM E18 Standard is used by 80% of laboratories worldwide.
3. Vickers Hardness (HV): It tests thin materials and coatings with high precision. What is its precision? The ISO 6507-1 Standard ensures a precision of ±1%.

(3)Factors that affect Steel Hardness

1. Chemical composition: Higher carbon content brings higher hardness. What is the hardness of Cr12 steel? After quenching and tempering, its hardness reaches HRC58-62 (based on ASM Heat Treatment Handbook).
2. Heat treatment: How does the hardness of 40Cr steel change? After quenching at 850℃ and tempering at 200℃, its hardness is HRC55-58. What about toughness? When tempered at 500℃, its toughness increases by 4 times (based on JHTA data).
3. Cold working: How does the hardness of low-carbon steel change after cold rolling? After 50% cold rolling, its HRB hardness rises from 60 to 95 (based on World Steel 2024 Report).

(4)Testing methods for Steel Hardness

There are three common methods for testing steel hardness. Each method corresponds to different scenarios and has clear standards.

The first method is the “Brinell Hardness Test” (HB). It follows the ISO 6506-1 Standard and is suitable for medium-low hardness steel (such as machine tool castings). You use a steel ball with a 10mm diameter, apply a 3000kgf pressure to press it on the steel surface, keep the pressure for 15 seconds, then remove the pressure. You measure the diameter of the indentation and calculate the hardness value using a formula— the larger the indentation, the lower the hardness.

The second method is the “Rockwell Hardness Test” (HR). It follows the ASTM E18 Standard and is suitable for steel of different hardness levels. You use a diamond cone (for testing high-hardness materials like cutting tools, with the HRC scale) or a steel ball (for testing medium-low hardness materials like cold-rolled steel, with the HRB scale).

After applying different pressures, you measure the indentation depth and directly get the hardness value. This test is fast and leaves small indentations, so it is suitable for batch testing.

The third method is the “Vickers Hardness Test” (HV). It is based on the ISO 6507-1 Standard and is suitable for precision scenarios like thin steel plates and surface coatings. You use a diamond square pyramid indenter, apply a small pressure to press it into the steel, measure the length of the indentation’s diagonal, and calculate the hardness. Its precision reaches ±1%.

In addition, the ISO 18265 Standard also specifies hardness conversion relationships (for example, HRC60 ≈ HB601), which facilitates the comparison of different test results. For example, when testing bearing steel, you can use Rockwell hardness to quickly judge whether its wear resistance meets standards.

How to Balance Strength, Toughness and Hardness in Industrial Production?

(1) The relationship between the three

1. Strength and toughness: They are mostly negatively correlated. How does impact toughness change when tensile strength increases? When tensile strength rises from 800MPa to 1500MPa, the impact toughness at -40℃ drops from 80J to below 25J (based on BAM research).
2. Strength and hardness: They are positively correlated. Higher hardness means higher strength. What can people do with this? They can quickly infer strength through hardness.
3. Hardness and toughness: They are negatively correlated. Higher hardness makes steel more brittle. What is needed? People need to balance the two.

(2) Practices for performance balance

1. Engineering machinery buckets: How to balance performance? The blade edge is welded with high-hardness alloy to resist wear, and the main body uses high-strength and tough steel to resist impact.
2. Automotive drive shafts: What kind of steel is chosen? Steel with “medium strength + medium toughness” is selected. Why? To avoid fracture caused by excessively high strength.

Conclusión

Strength, toughness and hardness of steel are interrelated and restrictive. What do people need to do in industrial material selection? They need to balance the three according to specific scenarios. For example, toughness is emphasized in low-temperature scenarios, and hardness is emphasized for cutting tools. What will happen in the future? The development of new steel will further break the performance contradictions and promote efficient industrial development. What is the key to selecting materials accurately and ensuring production safety? Mastering the core knowledge of these three indicators.