Steel in Shipbuilding: An Explanation of Steel Grades, Strength, and Toughness

Steel used in shipbuilding must withstand high mechanical stresses while reliably performing under a variety of operating conditions. But what do designations such as Grade D, Grade E, DH36, or FH36 actually mean?

When you read these designations, the first thing you notice are the numbers and letters. But how do the different grades actually differ—and what can you tell from their designations?

What types of steel are used in shipbuilding?

Normal-strength and high-strength

steel grades

are

used in shipbuilding

. Normal-strength grades include Grade A, Grade D, and Grade E. They are used, among other things, for load-bearing components such as the hull and deck.

For areas of the fuselage subject to higher stress, higher-strength grades such as DH36, EH36, and FH36 are suitable. They can be used, for example, in deck and floor areas subject to higher loads or in other primary load-bearing structures.

The EH40 and E500/EH50 grades also offer even higher strength values. For fuselage structures subjected to corresponding stresses, such materials can be used, for example, in areas under particularly high loads.

However, a ship is not made of a single grade of steel. Different grades may be used within a single structure. The decisive factors are the extent of the stress placed on a component, the function it performs, and the operating conditions to which it is exposed.

For example, a component subjected to lower stress can be made from a standard-strength grade. If, on the other hand, a component must withstand higher stresses, a higher-strength grade such as DH36 or EH36 may be specified. If higher toughness at low temperatures is also required, a grade with correspondingly higher notched impact toughness requirements may be necessary within the same strength class.

This makes it clear that the steel grade is not selected based solely on the type of ship. Rather, the specific stresses to which the respective component is subjected are the decisive factor.

Why Strength Alone Is Not Enough for Steel in Shipbuilding

Two types of steel can have comparable strength and yet be suitable for different operating conditions. One of the key factors is how the material behaves at low temperatures.

This is exactly where toughness comes into play. It describes how much energy a material can absorb before it breaks. It is determined, among other methods, by the notched impact test.

The test temperature is an essential component of the respective material requirement. The lower the required test temperature, the more challenging it is to demonstrate cold toughness.

For steel used in shipbuilding, this means that higher strength alone does not determine whether a grade is suitable for a specific design or service condition. Strength and toughness must be considered together.

What do Grade D, Grade E, DH36, and FH36 mean?

For Grade A, Grade D, and Grade E, the minimum yield strength is 235 MPa. The differences become apparent in the notched impact test:

• Grade A: +20°C
• Grade D: -20°C
• Grade E: -40°C

For DH36, EH36, and FH36, the minimum yield strength is 355 MPa. The number 36 indicates the strength class. The letters distinguish the requirements for notched impact toughness:

• DH36: -20°C
• EH36: -40°C
• FH36: -60°C

This means that grades within the same strength class can meet different requirements for toughness at low temperatures.

For EH40, the minimum yield strength is 390 MPa. E500/EH50 reaches 500 MPa and thus belongs to an even higher strength class.

Two factors are therefore particularly important when classifying a grade: What is the required strength, and at what temperature must the required toughness be demonstrated?

Conclusion:

Steel for shipbuilding is not selected based on a single criterion. Depending on the component and operating conditions, different requirements for strength and toughness may apply.

This is precisely why different grades of steel are used in ship construction. The grade designation provides important information about the requirements for which the material is designed.

The actual grade used depends on the specific design, the loads, the sheet thickness, the operating conditions, and the requirements of the applicable classification rules.