Weldox, which is a registered trademark for high strength steels from the Swedish company SSAB, is one of the most known High Strength Steels (HSS) on the market and has been in use for more than 20 years . 

The history of HSS starts in the 60's with yield strengths above 690 MPa. The first uses for these steels were military applications. The steels were alloyed to reach the mechanical properties and hence very difficult to weld at that time and involved careful control of  preheat, minimum interpass temperature, holding times at higher temperatures and controlled cooling rates.  

Modern HSS are to a much lesser extent dependent on the alloying elements for the strength properties. While Mn, Cr, Ni and Mo are still present as alloying  elements they are at lower levels. Today the steels are produced with a thermo mechanical process in the rolling line (TMCP = Thermo Mechanically Controlled Processed). The temperature history during the rolling process is carefully controlled to give the steel its properties through hardening and tempering processes during rolling.   

Different grades and carbon equivalents

Strength levels start at 690 Mpa and go upwards with grades as follows,  Weldox 700, 900, 960, 1030, 1100 and 1300. They are all readily weldable with  carbon equivalents ranging from 0.39 to 0.75. The usage for HSS is increasing steadily with the steel in many different applications from crash zones in cars, cargo supports in trucks, pipes, node sections in offshore industry and much more. 

A typical application where the importance of high strength and low weight is essential for the application

It is interesting to see how the steels have developed over the years both in the chemistry, strength levels and new applications. In the automotive industry the use of HSS has slowed down the introduction of aluminum. HSS steels have been developed with good formability and weldability with the result that they have become attractive in automotive applications requiring strength and low weight.

Consumables for HSS  

For the Weldox steels and also for general HSS it is possible to use low-alloyed (under matching), low-alloyed (matching) and stainless steel consumables. This might sound confusing that so many consumables can be used, but depending on the design of the welded object and the position of the joint this is possible. It is not so common that a matching consumable is necessary. Very often it is possible to place the joint in an area where the stresses are low and an under matching consumable can be used. The consumables should be chosen in accordance with the recommendations from the steel supplier or consumable supplier. Using a low strength consumable can offer several benefits, such as higher toughness of the weld metal, higher resistance to hydrogen cracking and lower residual stresses in the welded joint. 

In a multi-pass joint in HSS with yield from 700 to 1300 Mpa it is a benefit to use consumables of different strengths in the same joint. The technique is to use a low strength consumable for the tack welds, the root run and hot passes and depending on the thickness of the joint, 2 to 3 more passes, then fill and cap with a high strength consumable. This strategy  can increase both the toughness and the resistance to hydrogen cracking.     

Hydrogen cracking 

HSS can be sensitive to hydrogen cracking in general and therefore it is important to use the right preheat and interpass temperatures to avoid hydrogen cracking. Exact data will be found in the steel data sheets. Weldox steels in general show good resistance to hydrogen  cracking and susceptibility can be  further reduced by following these simple rules:

• Use the right preheat and interpass temperature 
• Use welding consumables with a low hydrogen content 
• Keep impurities out of the joint area 
• Do not use consumables of a higher strength than necessary 
• Arrange the weld sequence so that the residual stresses are minimized 
• Never use a root gap of more than 3 mm 

Most weld metals derive their strength from  microstructures, such as martensite, through alloying elements at relatively high levels. This means that the martensite transformation will take place at low temperatures and therefore it is recommended that for welding the steels with yields between 900 and 1300 Mpa  the interpass temperature be kept  below 175 deg.C.   

Hybrid Laser Arc Welding (HLAW)

As the HSS are sensitive to reduced strength in the HAZ due to too high heat input, it is advantageous to look at processes that induces less heat input to the steel. Hybrid Laser Arc Welding (HLAW) is a process that has been in industrial use for a decade and is a combination of laser and GMAW processes. The Laser process gives deep, narrow penetration at a high welding speed. While the GMAW process gives gap bridging properties and the opportunity to adjust  the metallurgy in the joint.  

The optics for the laser and the GMAW torch combined in a joining head that marries the focal point of the laser and the aiming point for the MAG wire.

The process equipment consists of conventional GMAW equipment and a Laser source that can be a CO2 laser, a Nd-YAG laser, a disc laser or a fiber laser. The choice of laser source will decide the complexity of the welding equipment. For mechanized or automated welding the best choice is to choose a laser where the beam transport can be done in an optical fiber. In the welding head the two processes are "married" and are carried by a robot or other equipment.

The benefit with this process is that high welding speeds can be achieved at the same time as a consumable is added to influence the properties in the weld-pool and also increase the heat input to the process. A laser by itself results in low heat input and too high hardness values in the HAZ.

Tests in Weldox 1100 with HLAW

In order to find the optimal consumable and to test if the process is applicable to high quality welds in HSS a number of trials were made to evaluate the properties. The steel used was of the following thicknesses and properties: 

In order to evaluate consumables according to the earlier statement that many different consumables can be used it was decided to test a Ni/Cu alloyed wire, a Cr/Ni/Mo alloyed wire, a 18Cr/8Ni alloyed wire and a Ni-base wire. The diameter of all wires was in this case 1,0mm.

The following table shows the names of the wires and also their composition and mechanical properties. 

Joint preparation for the 4 mm plate was an included angle of 2 degrees, zero gap.  For the 8 mm joint, zero gap, 4mm landing and 5 degree included angle was used. For the 12 mm joint zero gap a landing of 6 mm and a 5 degree included angle was used.

The results of the mechanical tests of the welded joints with the different consumables showed that there were small difference between the OK 13.13 and OK 13.26 and the only consumable that did not meet matching properties was the Ni-base wire OK 19.85. This is logical since it is a fully austenitic structure that can "absorb" dilution without any significant influence on the original mechanical properties of the pure weld metal. 

This table shows the result of the OK 13.26 wire.

This table shows the result of the OK 16.95 wire.

This table shows the result of the OK 19.85 wire, the Ni-base.

Noticeable here is that the mechanical properties of the joint do not match the base material but the requirements on the joint in this case was that the impact properties met   -40 deg.C min 55 J. And there are no commercial products on the market that both fulfils min 1100 Mpa yield and -40 deg.C impact properties. But what it clearly shows is the influence of the dilution from the base material on the weld metal properties, as all weld metal properties exceeds the pure weld metal properties.

A very important benefit with the Hybrid Laser process in welding of HSS are that a minimum of joint preparation is needed. In thicknesses of 4- 6 mm no preparation is necessary except the slight deviation that comes from the cutting process. In thicknesses above 6mm an included joint angle of 5 degrees is enough. The adding of a consumable is positive to bridge gaps in the joint and to influence the chemistry and thereby the properties of the welded joint. It results in very high welding speeds. Deformations are also significantly reduced and in some cases eliminated.

Another very important advantage is that a full penetration can be reached in many joints and thereby influence the fatigue properties. It will also result in the possibility to utilize the strength of a HSS in a joint to its full capacity.

The above photo illustrates the "normal" way of a T-joint. It contains a calculated root defect as a conventional welding process like GMAW cannot penetrate the full thickness.

This results in the fillet size and plate thickness having to be increased to carry the load applied on the joint.

On the other hand if the HLAW process is chosen, there is a possibility to have full penetration, welded ONLY from one side. This increases the strength of the joint and the fillet size can be reduced and also the plate thickness can be reduced. This way the weight of a construction can be reduced and economical benefits achieved.

The following photo shows a joint in a 16 mm thick pipe (the longitudinal seam) in material X80. It is welded with HLAW using a CO2 laser and a normal unalloyed wire. The joint preparation is zero gap, 7mm landing and an 8 degree include angle. This was done with a welding speed of 1 m/min and the total welding time for a 12 m long pipe was 18 min.

To do it in the "normal" way with a conventional joint preparation and a root-run with a hot-pass and then fill and cap takes app. 4.5 hrs.

The time saving in this application has enormous implications on the production costs and through put capacity and justifies the investment in the laser equipment.

Conclusion   

The usage of the HLAW process will in the joining of plates in the thickness range from 8 - 40+ mm in the future. Lasers will be cheaper in the future making the process even more economical. But it is important that the right applications are chosen because as with all other processes in welding it is important to choose the right process to the right application!