Based on our experience the number one cause of fatigue failures in trailer chassis is due to poor weld quality. Therefore, it is important, not only to thoroughly evaluate the new upgraded design, but also to secure the production process. Poor weld or edge quality will rapidly decrease the service life of any trailer.
Edge quality
Edge quality is also important for fatigue resistance and different cutting methods result in different edge quality. The graph shows the results of fatigue testing of Strenx® 700 MC where the plate edge has been milled, laser-cut and sheared. Laser-cut edges are often better than mechanically cut edges. To achieve good fatigue strength of mechanically cut edges it is important to remove all visual crack-like defects.
Modern thermal cutting processes like laser or high definition plasma generally produce edges in Strenx® 700 MC with good fatigue properties. The ranking of such methods from a fatigue point of view would be laser, plasma and manual gas-cutting. To avoid fatigue problems it is also important to place start and stop-positions in low stress areas. Blasting of a structure with cut edges normally has a positive effect on fatigue resistance.
Results from fatigue testing of Strenx® 700 MC with different edge conditions.
Hot dip galvanizing
Piecewise hot dip galvanizing is used in order to improve the corrosion resistance. Strenx® High Strength Steels up to Strenx® 700 MC are very well suited for hot dip galvanizing if ordered in a specific condition with a certain Si-content. The mechanical properties of the steels are generally not influenced by the galvanizing process; for instance there is no influence on the fatigue properties of welded joints. However, it could be noticed that the fatigue strength of the base material is slightly reduced due to the zinc layer and the interface between the zinc layer and the steel surface. For Strenx® 700 MC hot dip galvanizing typically reduces the fatigue strength of the base material by about 10 %.
Welding methods
All conventional welding methods can be used for AHSS. The most commonly used welding methods in the trailer industry today are:
- MAG-welding (with solid or cored wire)
- Submerged Arc Welding (sometimes used to produce the longitudinal beams)
Filler metal
The strength of the filler material should normally be matched to the strength of the base material, according the table. However welds in trailer applications are very seldom subjected to stress levels that require matching filler metal. In most cases it is therefore possible to use undermatching filler metals. If Submerged Arc Welding is used for welding of AHSS, basic flux is recommended.
| Steel Grade | MAG-welding (GMAW) | Submerged Arc Welding (SAW) |
| Strenx® 700 MC | AWS: A5.28 ER100S-X | AWS: A5.23 F10X |
| AWS: A5.28 ER110S-X | AWS: A5.23 F11X | |
| EN 12534: G Mn3Ni1CrMo | EN ISO 26304-A S69X | |
| EN 12543: G Mn4Ni2CrMo | ||
| Docol® 1000 DP Docol® 1200 M |
AWS: A5.28 ER110S-X | |
| AWS: A5.28 ER120S-X | ||
| EN 12534: G Mn3Ni1CrMo | ||
| EN 12543: G Mn4Ni2CrMo |
Filler material for Strenx® and Docol® AHSS steels.
Welding heat input
Strenx®
High strength steels are somewhat sensitive to high heat inputs. Excessive high heat input decreases the strength as well as the impact toughness of the welded joint. If the minimum yield strength of the base material must be fulfilled in the welded joint, the maximum recommended heat input should be exceed the values in the graph.
The graph is valid for butt welds, welded with matching filler material, and where the reinforcement has been removed before testing. The maximum interpass temperature 100 °C.
The heat input can be calculated according to:
Where Q = Heat input (kJ/mm), U = Voltage, I = Current, v = Travel speed (mm/min), k = Arc effciency.
The arc effciency values are 0.8 for MAG welding and 1.0 for SAW welding. If the weld is located in a low stressed area and the impact toughness requirement is of lesser importance, higher heat inputs can be used.
Docol®
Docol® AHSS are produced in thinner gauges, which makes it more difficult to decrease the heat input to such an extent that the heat affected zone can be limited to a level, where a base material failure can be achieved. As a rule of thumb: weld with as low a heat input as possible.
Welding distortion
Strenx®
In practice for trailer applications, distortion due to welding is more critical than the static strength of the welds. In order to minimize the amount of distortion due to the welding operation:
- Weld with as low a heat input as possible
- Minimize the cross sectional area of the weld, see illustration
- Prebend, clamp or angle the parts before welding in order to compensate for the shrinking
- Avoid irregular gaps in the root
- Use symmetrical welds, see illustration
- Minimize reinforcements and optimize the throat thickness of the fillet welds
- Weld from rigid areas to loose ends
- Optimize the welding sequence
Cross-section of the weld and how it influences the angle deviation.
Use a symmetrical welding sequence.
In order to avoid a curved/deformed longitudinal beam after welding of the web, displace the web in relation to the center of the flange. This makes it possible to locate the longitudinal welds in the neutral layer of the flanges.
Displacing the web in relation to the flange makes it possible to weld in the neutral layer of the flange, which avoids deformations from welding.
Docol®
In order to avoid heavy deformation on Docol® steels the following is recommended:
- Weld with as a low heat input as possible
- Use a wire with a small diameter (0.8 mm).
- Downhill welding technique reduces the heat input and is recommended if permitted by the application and the production conditions.
- If the application allows a small gap between the sheets, intermittent welding can be used.
- If the application requires a sealed connection, silicon or adhesives could be used instead of welding to seal the connection and prevent corrosion to arise.
- Use lap welds instead of butt welds
- Welding of thin sheets requires a short distance between the tack welds.
- Power sources for MAG-welding have developed
- a lot lately which makes it possible to weld thin sheets with approximately 50 % lower heat input.
- If the joint is accessible from both sides resistance spot welding can be used instead of fusion welding.
- Avoid welding and use mechanical joining which guaranties very low or no deformation.
Hot straightening
Hot straightening is a very common method to restore longitudinal beams for trailer chassis that have been distorted due to welding. It is not recommended to use hot straightening for Strenx® and Docol® AHSS. It is not recommended to use hot straightening for Strenx® and Docol® AHSS. This is because the steel may lose its guaranteed properties in the heated area.
The recommended maximum temperature these steels can be subjected to without loosing their guaranteed mechanical properties is:
- Strenx® 700 MC: 650 °C
- Docol® 1000 DP: 300 °C
- Docol® 1200 M: 200 °C
Weld quality
If the flanges of the longitudinal beam have to be welded (which is not recommended) in order to increase the length of the flange or beam use a temporary sacrificial plate for critical weld starts and stops. The temporary plate can easily be removed by grinding after welding.
Pay attention to the stops, which are placed on the sacrificial plate. This plate is then removed by grinding.
Grind the edge with a smooth grinding wheel and make sure that the direction of the grinding scratches is longitudinal to the loading direction. Another alternative is to design the weld joint at a 45 degree angle to the length of the flange in order to avoid a principal stress flow in the trans- verse direction of the weld.
Welding with rutile cored wires has the tendency to create very smooth transitions between the weld metal and the base material. This property could be used in order to increase the fatigue performance of the weld, especially in critical areas like the landing gear attachment and the hanger bracket attachments.
In order to secure the weld quality it is important that all welds are examined. Defects that are especially detrimental for the fatigue performance of the trailer are surface breaking weld defects such as:
- Undercuts
- Root defects
- Lack of fusion
- Cold laps
- All types of cracks
Proper direction of the grinding scratches oriented longitudinal to the stress.
With a proper welding technique and knowledge about how these defects occur is it possible to avoid unnecessary repairing. The tables below describes the most common weld discontinuities, how they occur and actions to avoid them.
Welding defects, why they arise and actions to avoid them: a) lack of penetration b) undercut c) lack of fusion d) pipe e) porosity f) sagging.
The information in this report is only applicable to SSAB’s products and should not be applied to any other products than original SSAB products.
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