Part 5: Welding and fabrication of High-Strength Steel for vehicle structures

Replacing conventional steel with High-Strength Steel does not mean that the load-carrying capacity of the entire structure will automatically increase by the same proportion. In vehicle structures subjected to dynamic loading, welds and the heat-affected zone still require close control.

When the steel grade changes, the fabrication process also needs to be reviewed, including welding consumables, preheating, interpass temperature, joint design and bending direction.

High-Strength Steel can therefore deliver its intended benefits only when the material, structural design and fabrication process are treated as one integrated system.

1. The weld is part of the load-bearing structure

In dynamically loaded structures, welds are often located where geometry changes and stress concentrations occur.

If the base steel has a high yield strength but the welded joint does not meet the design requirements, the mechanical-property advantage of the base material cannot be fully translated into the performance of the complete structure.

The following factors need to be considered together:

base steel grade
joint type
welding consumables
thickness
load transferred through the weld
dynamic loading conditions
welding procedure
fabrication sequence

The objective is not to give the weld the highest possible mechanical properties. The objective is to ensure that the welded joint meets the load-carrying, fatigue and fabrication requirements defined by the design.

This is also why welding consumables should not be selected simply according to what is already available in the workshop.

2. Do not use the same welding guidance for High-Strength Steel and wear-resistant steel

High-Strength Steel and wear-resistant steel may both have high mechanical properties, but they are developed for different functions.

Each material group therefore needs to be welded according to the documentation and procedure applicable to the specific product being used.

Item	Point requiring control
Preheating	Depends on steel grade, thickness and welding conditions
Welding consumables	Need to meet the requirements of the joint and structure
Interpass temperature	Needs to remain within the range defined by the procedure
Crack resistance	Depends on material, hydrogen, HAZ hardness and stress state
Welding quality requirements	Need to correspond to the load-bearing role of the structure

A welding instruction developed for wear-resistant steel should not be applied directly to High-Strength Steel simply because the two plates look similar.

Conversely, a welding procedure for a load-bearing structure should not automatically be applied unchanged to a wear-resistant lining plate.

3. Determine preheating conditions

Preheating is not based on one fixed temperature for every steel grade.

The requirement depends on several factors, including at least:

steel grade
thickness at the joint
chemical composition and carbon equivalent
type of welding consumable
diffusible hydrogen content
degree of restraint
environmental conditions
welding heat input

EN 1011-2 can be used as a reference when determining welding conditions for ferritic steels, including preheating and control of hydrogen cracking risk.

The appropriate approach is therefore not “this is the temperature the workshop has always used”, but:

Which grade → What thickness → Which joint → Which procedure

4. Temperature needs to be controlled before and during welding

Checking the temperature once before striking the arc is not sufficient.

During fabrication, the material temperature can change after each weld pass or during the waiting period between passes.

Two temperature levels need to be controlled.

Preheat temperature

This is checked before welding begins to confirm that the material has reached the condition required by the procedure.

Interpass temperature

This is monitored during welding to ensure that the temperature remains within the specified range.

Item	Method
Measuring equipment	Contact thermometer or other suitable measuring device
Inspection timing	Before welding and between weld passes
Measurement position	According to the procedure applicable to the joint
If outside the required range	Stop work and return the temperature to the specified range before continuing

Temperature-indicating crayons may support quick checks, but where numerical records are required, equipment capable of providing clearer measurement and recording should be used.

More importantly, the measured values need to be recorded rather than relying only on verbal confirmation in the workshop.

5. Welding consumables need to match the requirements of the joint

Welding consumables should not be selected simply by matching the grade designation of the base steel.

The following need to be considered together:

load requirements of the joint
required yield strength and tensile strength
required toughness
dynamic loading conditions
approved welding procedure
steel manufacturer’s recommendations
standard applicable to the structure

In some designs, welding consumables may be selected to provide properties comparable to the base steel. In other cases, consumables with lower or higher mechanical properties may be deliberately used if permitted by the design and welding procedure.

A safer principle is therefore:

Welding consumables need to meet the requirements of the joint and be defined in the welding procedure, rather than being selected according to individual workshop experience.

Distinguishing this from wear-resistant lining plate

For some wear-resistant lining plates, the weld primarily serves to hold the plate in its operating position rather than carrying structural loads in the same way as a beam or frame.

The principles used to select welding consumables may therefore be different.

Before selecting the consumable, it is necessary to determine:

Is the weld transferring structural load, or is it primarily fixing a lining plate in position?

The same selection rule should not be applied to both cases.

6. Changing the steel grade may also require different welding consumables

When moving from a lower steel grade to a higher grade of High-Strength Steel, the requirements for welding consumables may also change.

According to the welding-consumable recommendations by steel grade in the manufacturer technical documentation used for the perform® range, changing the steel grade may require a different consumable group.

Reference steel grade range	General trend in recommended welding consumables
Around 315 MPa	Conventional carbon steel welding consumables
Around 355-420 MPa	Higher-strength welding consumable grades
From around 460 MPa	Nickel-alloy welding consumables may be required
From around 550 MPa	Suitable nickel-molybdenum alloy systems may be required

This table is included only to show that welding consumables may change with the steel grade. Actual application needs to follow the correct documentation for the steel grade, welding consumable, thickness and welding procedure being used.

This affects both technical planning and procurement.

Changing the steel grade may require changes to:

welding consumables
storage requirements
supply lead time
inventory
welding procedure
welder requirements

The steel grade and welding solution should therefore be defined together during the design stage rather than treated as two independent decisions.

7. Verify the welding procedure and welding capability before fabrication

For load-bearing structures, welder experience is important, but it does not replace technical documentation.

Before fabrication, the following items should be checked as appropriate to the scope of work:

approved welding procedure
scope of procedure qualification
welder qualification
material group
welding process
welding position
welding consumables
validity of relevant qualification records

EN ISO 3834-2 is referenced in this knowledge series for comprehensive quality requirements for fusion welding of metallic materials.

The standard does not replace the drawing or the specific welding procedure, but it provides a quality-control framework for welding activities.

What should be avoided is allowing fabrication to proceed solely on the basis that “the welder has worked with this type of steel for many years” without checking whether the qualification records actually cover the work being performed.

8. Weld design needs to consider dynamic loading and fatigue

A weld that can carry static load is not necessarily a suitable detail for repeated loading.

For vehicle structures, fatigue needs to be considered from the connection-layout stage.

Three principles require particular attention:

Principle	Significance
Minimize unnecessary weld terminations in high-stress areas	Weld ends can create stress concentration locations
Control weld-toe geometry	The weld toe is sensitive to fatigue crack initiation
Reduce the number of welded connections in dynamically loaded areas where the design allows	Reduces the number of locations requiring fatigue control

This does not mean that every dynamically loaded area requires one specific weld type. Joint type, weld length and geometric details need to be determined according to the design and applicable standard.

Adding stiffeners does more than increase stiffness

Adding stiffeners can:

reduce the effective plate span
reduce deflection
improve local stability

But it can also:

increase mass
increase total weld length
create additional heat-affected zones
create additional locations requiring fatigue control

By contrast, changing the cross-sectional geometry may improve geometric stiffness without necessarily increasing the number of welds by the same proportion.

The choice between adding stiffeners and changing the cross-sectional geometry therefore needs to be made during the design stage rather than waiting until the workshop discovers excessive deflection and then adding reinforcement.

9. Do not evaluate weld quality only by external appearance

A smooth, visually attractive weld with little spatter does not demonstrate that the entire welding process was performed correctly.

Joint quality also depends on:

edge preparation
welding consumables
preheat temperature
interpass temperature
heat input
welding sequence
internal imperfections
heat-affected zone

In particular, inspection should not focus only on the weld metal. The material immediately adjacent to the weld also needs to be considered.

If a crack is found running along the region beside the weld, the surface should not simply be ground out and covered with a new weld.

The cause needs to be investigated, including:

thermal procedure
hydrogen
welding consumables
stress state
joint configuration
fabrication sequence

Treating the visible indication without identifying the cause may allow the crack to reappear.

10. Rolling direction needs to be included on fabrication drawings

The bendability of High-Strength Steel depends on:

steel grade
thickness
bending radius
bending direction relative to the rolling direction
forming equipment and method

Bending in different directions relative to the rolling direction should not automatically be assumed to use the same minimum bending radius.

If bending direction affects the fabrication requirement, it should therefore be shown directly on the drawing.

The workshop should not rotate the plate into whichever orientation is most convenient if the design depends on the rolling direction.

This becomes even more important when formability is being used to:

create deeper sections
reduce the number of stiffeners
increase the moment of inertia
reduce the number of welds

The material’s formability provides a real benefit only when the drawing and blank-cutting process maintain the orientation used in the design.

11. Items to check directly in the workshop

Item	Inspection method	Action if not compliant
Preheat temperature	Measure with suitable equipment	Stop and return to the required range
Interpass temperature	Measure during welding	Adjust before continuing
Welding consumables	Verify material code against documentation	Replace with the approved type
Welder qualification	Check validity and scope	Do not assign the work if the qualification is not suitable
Bending direction	Compare the drawing with the rolling direction	Do not continue if the orientation is incorrect
Weld geometry	Visually inspect and measure using suitable tools	Correct according to the acceptance criteria
Material identification number	Compare the plate marking with the records	Clarify before further fabrication

These inspection points should be defined in advance in the quality plan or fabrication documentation.

Checks should not be postponed until the entire structure has been assembled, particularly where information may be lost during fabrication.

12. Material traceability needs to be maintained after fabrication

After a steel plate is cut into multiple smaller components, the original identification mark may be lost.

Without a method for transferring and retaining identification information, it can become difficult to determine which plate or batch a finished component came from.

Fabrication records should therefore retain at least:

material identification number
location of use within the structure
material grade and thickness
welding consumable batch
person performing the work
applicable welding procedure
key inspection points
weld photographs where required

Traceability is not only required for acceptance inspection.

If a crack or deformation appears after a period of operation, this information allows the technical team to identify the exact material and procedure used at that location.

13. Issues that need to be resolved before completing fabrication release documentation

The structure should not be released while important information remains unresolved, such as:

missing material identification number
welding consumable used cannot be identified
missing temperature records where temperature control was required
welder qualification does not cover the work
bending direction does not match the drawing
signs of cracking with an unidentified cause
weld inspection results remain unresolved
missing records for mandatory inspection points

The objective is not to create more paperwork. It is to ensure that critical information can still be verified after the structure has been completed.

14. Responsibilities of the different functions

Task	Responsible person or function
Define load requirements at the joint	Structural design team
Develop or approve the welding procedure	Authorized welding engineering function
Select welding consumables	Technical team according to the design and procedure requirements
Show bending direction on the drawing	Design team
Verify welder qualification	Quality function or workshop supervisor according to procedure
Measure and record welding temperatures	Personnel assigned under the procedure
Inspect materials before fabrication	Quality department
Maintain material traceability	Workshop in coordination with the quality department
Resolve cracking indications or deviations	Technical function responsible for approving the solution

Welders need to work according to the defined procedure and should not independently change the welding consumables, thermal parameters or joint configuration for convenience.

15. Notes on applying this knowledge

First, preheat and interpass temperatures depend on the steel grade, thickness, joint conditions and welding procedure. There is no single value that applies to all High-Strength Steel.

Second, welding consumables need to be selected according to the requirements of the joint and the approved procedure. Welding consumables should not automatically be assumed to require a higher grade than the base steel, and welding rules for wear-resistant plate should not be transferred directly to load-bearing structures.

Third, changing the steel grade may require changes to welding consumables, procedures, storage and fabrication capability. These factors need to be considered before the material solution is finalized.

Fourth, a visually attractive weld is not sufficient evidence of quality. The heat-affected zone, thermal parameters, welding consumables and imperfections need to be inspected according to the requirements of the structure.

Fifth, adding stiffeners can improve deflection and stability but also increases the number of welds. For dynamically loaded structures, the effect on fatigue needs to be considered from the design stage.

Sixth, rolling direction and bending direction need to be controlled where the forming solution depends on them. Plate orientation should not be decided only after the drawing has been completed.

Seventh, this article identifies the key controls for welding and fabrication of High-Strength Steel. It does not replace the welding procedure, structural calculations or acceptance criteria for a specific project.

When changing to High-Strength Steel, the change does not stop at the steel plate itself. Welding consumables, temperature control, joint geometry, bending direction and traceability all need to follow the new design solution.

A properly controlled fabrication process preserves the benefits of the material in the completed structure instead of losing them at the joints and during fabrication.