Wind Turbine Tower Steel Plate: S355 Buying Guide

Wind Turbine Tower Steel Plate: S355 Buying Guide

A wind turbine tower may look like one large steel structure, but its shell
plates, flange zones, door-frame reinforcements and offshore foundation
sections do not necessarily use the same steel quality or plate thickness.

For international buyers, specifying only
“S355 steel plate for wind tower”
is usually not enough.

The purchase order may also need to define:

• the exact S355 quality;
• delivery condition;
• impact toughness;
• plate thickness and width;
• through-thickness properties where required;
• ultrasonic testing;
• welding requirements;
• inspection certificate;
• full heat-number traceability.

This guide explains how to distinguish S355J2+N, S355NL and S355ML when
buying heavy plate for wind turbine towers and related wind-energy structures.

Quick Answer: Which S355 Grade Is Used for Wind Turbine Towers?

There is no single universal S355 grade for every wind tower.

Depending on the approved tower design, plate thickness, location and
temperature requirements, wind-energy projects may use materials such as:

• S355J2+N;
• S355NL;
• S355ML;
• higher-strength fine-grain structural steels;
• offshore-specific structural grades for foundations and support structures.

Actual offshore wind projects have used combinations of S355J2+N, S355NL,
S355ML and offshore structural grades rather than one universal plate grade.

The correct material therefore comes from the approved material specification
and drawing—not simply from the fact that the structure is a wind turbine.

Where Is Heavy Steel Plate Used in a Wind Turbine?

Heavy plate is used primarily in the tower and, for offshore projects,
in large support and foundation structures.

Typical fabricated products can include:

• tower shell courses;
• rolled cylindrical cans;
• tower base sections;
• flange-adjacent shell sections;
• door-frame reinforcement plates;
• internal structural plates;
• transition pieces;
• monopile sections;
• offshore substation structures.

The plate requirements can change significantly between these parts.

A thin upper tower shell and a thick lower tower section should not be assumed
to have identical mechanical, toughness or through-thickness requirements.

S355J2+N vs S355NL vs S355ML

These materials belong to the same general S355 strength family, but they
should not be treated as interchangeable product names.

The steelmaking and rolling route, toughness quality, chemical composition
limits and applicable product standard can differ.

What Does S355J2+N Mean?

S355J2 is a non-alloy structural steel quality under EN 10025-2.

The J2 designation identifies a specified Charpy impact-toughness quality
at a lower test temperature than JR or J0.

The additional:

+N

identifies the specified normalized or normalized-rolled delivery condition.

Therefore:

S355J2

and:

S355J2+N

should not automatically be treated as identical purchase descriptions.

If the wind-tower drawing requires +N, that delivery condition should appear
clearly on:

• the RFQ;
• purchase order;
• material certificate;
• plate identification.

What Is S355NL?

S355NL belongs to the EN 10025-3 family of normalized or normalized-rolled
weldable fine-grain structural steels.

Fine-grain structural steels are particularly relevant to large welded
structures where a combination of:

• strength;
• toughness;
• weldability;
• low-temperature performance;
• heavy-section fabrication

is important.

The “L” quality indicates a lower-temperature toughness requirement than
the corresponding N quality.

For this reason, S355NL may be specified for wind-energy structures that
require improved toughness at lower temperatures.

However, the project engineer must determine whether S355NL is required.
A supplier should not change S355J2+N to S355NL simply because it appears
to offer better toughness.

What Is S355ML?

S355ML is a thermomechanically rolled, weldable fine-grain structural steel
under EN 10025-4.

The designation combines:

• S — structural steel;
• 355 — S355 strength class;
• M — thermomechanical rolling condition;
• L — specified lower-temperature impact quality.

Thermomechanical rolling can provide a useful combination of strength,
toughness and weldability for large welded structures.

S355ML has been used in actual offshore wind projects for towers,
monopiles and support structures.

However:

S355ML is not simply “S355J2 with better properties.”

The product standard and manufacturing route are different and must be
approved by the tower designer.

Why Plate Thickness Matters

Wind turbine towers use a range of plate thicknesses.
Lower tower sections and offshore foundations can require significantly
heavier plate than upper tower sections.

The number “355” should not be interpreted as a guaranteed minimum yield
strength of exactly 355 MPa for every plate thickness.

EN structural-steel standards apply thickness-dependent mechanical-property
requirements.

For procurement, buyers should therefore check:

• the exact plate thickness;
• the applicable standard table;
• minimum yield strength for that thickness range;
• tensile strength;
• elongation;
• impact-toughness requirement.

Do not copy mechanical values from a 10 mm datasheet into an order for an
80 mm or 100 mm heavy plate without checking the applicable requirements.

Wind Tower Shell Plates Need More Than Strength

A wind tower is a highly welded structure.

Plate selection therefore needs to consider:

• weldability;
• impact toughness;
• forming performance;
• plate flatness;
• thickness consistency;
• internal soundness;
• cut-edge quality;
• traceability.

Large shell plates are rolled into cylindrical sections and joined by
longitudinal and circumferential welds.

Poor plate flatness or inconsistent dimensions can increase:

• rolling difficulty;
• fit-up time;
• weld-gap variation;
• rework;
• fabrication cost.

This is why plate tolerances should be reviewed before production rather than
only after the material arrives.

For dimensional procurement guidance, see JUDE Steel’s

Carbon Steel Plate Tolerances Guide
.

When Are Z25 or Z35 Through-Thickness Properties Required?

Some highly restrained welded joints can create tensile stress through the
thickness of the steel plate.

Where the design identifies a risk of lamellar tearing, the project may
specify improved through-thickness ductility according to EN 10164.

Common quality classes include:

These values relate to reduction of area in through-thickness tensile testing.

Not every wind-tower plate needs Z-quality.

It may be required for selected:

• thick flange zones;
• highly restrained welded joints;
• door-frame reinforcement details;
• other joints identified by the structural designer.

Do not add Z35 to every tower plate simply because it sounds like a higher
quality.

It adds a specific material requirement that can affect availability,
cost and lead time.

Ultrasonic Testing Is Not the Same as Z35

This distinction is important.

Ultrasonic testing can be used to assess internal discontinuities in plate.

Z-quality, by contrast, controls mechanical deformation properties in the
through-thickness direction.

A plate can pass ultrasonic inspection without automatically satisfying
Z35 requirements.

Likewise, specifying Z35 does not remove the need for any separate ultrasonic
inspection required by the project.

A professional RFQ should state both requirements independently when both
are needed.

Onshore Towers and Offshore Foundations Need Different Reviews

The phrase “wind turbine steel” covers several very different structures.

An onshore tower shell may be governed by one material schedule, while an
offshore monopile or jacket can require offshore-specific structural steels
and additional toughness, weldability and inspection requirements.

Do not automatically replace an offshore grade with a general EN 10025-2
S355 plate merely because the nominal yield-strength class is similar.

The applicable offshore design and material specification controls.

Why Actual Wind Projects Use More Than One Steel Grade

A large wind farm can contain multiple steel grades because different
structural zones solve different engineering problems.

Published offshore wind projects have used combinations of:

• S355J2+N;
• S355NL;
• S355ML;
• S420ML;
• offshore-specific S355G grades;
• other higher-strength structural steels.

That does not mean every new project requires this complete list.

It demonstrates why procurement should follow the approved material schedule
for each component rather than convert the entire project to one generic
“S355 plate” description.

Welding Requirements Should Be Reviewed Before Ordering

Wind-tower fabrication involves extensive welding, including long
circumferential and longitudinal seams.

Before selecting plate, the fabricator should review:

• plate thickness;
• carbon-equivalent requirements;
• welding process;
• heat input;
• preheat requirements;
• interpass temperature;
• hydrogen control;
• weld consumables;
• impact requirements in the weld and HAZ;
• NDT requirements.

A steel grade with good published weldability still requires an approved
welding procedure.

The material supplier should not select the WPS from the steel grade alone.

Plate Width Can Affect Tower Fabrication Economics

Wind-tower manufacturers often require large plate widths and lengths to
reduce the number of welds and improve fabrication efficiency.

Therefore, the buyer should not compare quotations using price per tonne alone.

Also compare:

• available plate width;
• maximum plate length;
• plate weight limits;
• number of required shell segments;
• cutting yield;
• rolling layout;
• additional butt welds;
• transport restrictions.

A lower-priced plate that requires additional weld seams can create a higher
finished-tower cost.

What Should Buyers Check on the MTC?

The Mill Test Certificate should match the exact line item in the tower
material schedule.

Depending on the project, verify:

• manufacturer;
• heat number;
• full steel designation;
• EN product standard;
• delivery condition;
• plate dimensions;
• chemical composition;
• yield strength;
• tensile strength;
• elongation;
• Charpy test temperature and values;
• Z-quality results where required;
• UT results where required;
• inspection certificate type;
• traceability to the physical plate.

A certificate stating only:

S355

is not sufficient evidence for an order requiring:

S355NL + Z35 + project-specific UT

For document review, see:

How to Read a Carbon Steel Mill Test Certificate
.

What Should a Wind-Tower Steel RFQ Include?

A professional wind-energy plate RFQ should include enough information for
the supplier to verify both material compliance and production feasibility.

Example RFQ: S355NL Plate for a Wind Tower

This is an illustrative purchasing format rather than a confirmed stock
offer or a material recommendation for every wind tower.

Common Wind-Tower Steel Purchasing Mistakes

• Ordering only “S355 plate” without the full quality designation.
• Assuming S355J2+N, S355NL and S355ML are interchangeable.
• Assuming 355 MPa minimum yield strength applies to every plate thickness.
• Adding Z35 to all plate line items without engineering justification.
• Assuming ultrasonic testing proves Z-quality.
• Using general structural steel automatically for offshore monopiles.
• Ignoring plate-width limitations before finalizing tower nesting.
• Comparing price per tonne without considering welding and cutting yield.
• Accepting an MTC that does not show the required delivery condition.
• Failing to preserve individual plate traceability after cutting.

Frequently Asked Questions

Which steel grade is commonly used for wind turbine towers?

S355-family structural steels are widely used, including grades such as
S355J2+N, S355NL and S355ML.
The exact grade depends on the approved tower design, plate thickness,
toughness requirements and project specification.

Is S355NL stronger than S355J2+N?

They belong to the same general S355 strength class.
The important differences include product standard, fine-grain requirements,
delivery route and toughness properties rather than a simple “stronger”
ranking.

What is the difference between S355NL and S355ML?

S355NL is supplied under the normalized/normalized-rolled fine-grain
structural steel standard.
S355ML is thermomechanically rolled.
Both can provide low-temperature toughness, but they are different material
specifications and should not be substituted without approval.

Does every wind-tower plate need Z35?

No.
Z35 is specified when the structural design requires enhanced
through-thickness ductility, typically in selected highly restrained or
thick welded details.

Is ultrasonic testing the same as Z35 testing?

No.
UT examines internal discontinuities.
Z35 refers to specified through-thickness tensile ductility.
A project can require one, both or neither.

Can ordinary S355JR be used for a wind turbine tower?

Only when permitted by the approved design and project specification.
Wind-energy structures can have specific toughness, thickness and delivery
condition requirements that are not defined by the S355 strength class alone.

Is offshore wind steel the same as onshore wind tower steel?

Not necessarily.
Offshore foundations can require dedicated offshore structural grades,
greater plate thickness, additional toughness and more demanding inspection
requirements.

Why is plate width important for wind tower fabrication?

Larger suitable plate dimensions can influence tower-shell nesting, the
number of weld seams, fabrication time and material yield.
The optimal plate size should therefore be considered together with the
approved tower design.

What should I send for a wind-tower plate quotation?

Send the full material specification, plate BOM, thickness, width, length,
quantity, toughness requirements, Z-quality where required, UT requirements,
inspection certificate, processing drawings and destination.

Request Steel Plate for Wind-Energy Projects

JUDE Steel supplies

carbon and alloy steel plate

for structural and industrial fabrication.

Wind-energy projects should be reviewed against the exact material schedule.
Specific S355J2+N, S355NL, S355ML, Z-quality, offshore grades, dimensions
and inspection requirements must be confirmed for the actual order.

JUDE Steel’s

quality-control information

provides additional background on incoming inspection, process control and
material documentation.

Technical note:
This article is intended for preliminary material selection and procurement
guidance.
Final wind-tower, monopile or offshore-structure material must follow the
approved structural design, applicable EN or offshore standard, fatigue and
fracture-toughness requirements, welding specification and project material
schedule.
The S355 designation alone does not establish suitability for every
wind-energy component.