Clean the weld area to remove moisture, oil, corrosion or any impurities prior to welding. In addition to good welding hygiene, make sure you consider the following aspects:
Strength of unalloyed and low-alloyed welding consumables
Unalloyed and low-alloyed consumables with a maximum yield strength of 500 MPa (72 ksi) are generally recommended for Hardox. Consumables of higher strength (Re max. 900 MPa/130 ksi) may be used for Hardox 400 and 450 in the thickness range 0.7 – 6.0 mm (0.028” – 0.236”). Low-alloyed consumables result in higher hardness of the weld metal which can reduce the wear rate of the weld metal. If the wear properties of the weld metal are essential, the top cap of the joint could be welded with consumables used for hardfacing; see the chapter “Hard facing” on page 15. In addition, recommended consumables for Hardox steels and their designations according to AWS and EN classifications can be found in Table 1 on page 6.
The hydrogen content should be lower than or equal to 5 ml of hydrogen per 100 g of weld metal when welding with unalloyed or low-alloyed welding consumables.
Solid wire used in MAG/ GMA and TIG/ GTA welding can produce these low hydrogen contents in weld metal. Hydrogen content for other types of welding consumables should be obtained from the respective manufacturers.
If consumables are stored in accordance with the manufacturer’s recommendations, the hydrogen content will be maintained to meet the requirement stated above. This also applies to all coated consumables and fluxes.
Consumables of austenitic stainless steels can be used for welding all Hardox products, as shown in Table 2. They allow welding at room temperature 5 – 20°C (41 – 68 °F) without preheating, except for Hardox 600 and Hardox Extreme.
SSAB recommends giving first preference to consumables in accordance with AWS 307 and second preference to those in accordance with AWS 309. These types of consumables have a yield strength of up to approximately 500 MPa (72 ksi) in all weld metal.
The AWS 307 type can withstand hot cracking better than AWS 309. It should be noted that manufacturers seldom specify the hydrogen content of stainless steel consumables, since hydrogen does not affect the performance as much as it does in unalloyed and low-alloyed consumables. SSAB does not impose any restrictions on the maximum hydrogen content for these types of consumables.
Shielding gases for Hardox wear plate are generally the same as usually selected for unalloyed and low-alloyed steels.
Shielding gases used for MAG/ GMA-welding of Hardox steels usually contain a mixture of argon (Ar) and carbon dioxide (CO2). A small amount of oxygen (O2) is sometimes used together with Ar and CO2 in order to stabilize the arc and reduce the amount of spatter. A shielding gas mixture of about 18–20% CO2 in argon is recommended for manual welding, which facilitates good penetration in the material with a reasonable amount of spatter. If automatic or robot welding is used, a shielding gas containing 8–10% CO2 in argon could be used in order to optimize the weld result with regards to the productivity and spatter level. Effects of various shielding gas mixtures can be seen in Figure 1. Recommendations for shielding gas in different welding methods can be found in Table 3. Shielding gas mixtures mentioned in Table 3 are general mixtures that can be used for both short-arc and spray-arc welding.
In all welding methods based on shielding gas, the flow of shielding gas depends on the welding situation. As a general guideline, the shielding gas flow in l/min should be set to the same value as the inside diameter of the gas nozzle measured in mm.
Heat input (Q) is the amount of energy applied to the base material per length unit. Heat input is calculated according to the formula below:
Various welding processes have different thermal efficiency. Table 4 describes approximate values for different welding methods.
Excessive heat input increases the width of the heat affected zone (HAZ), which in turn impairs the mechanical properties as well as the wear resistance of the HAZ. Welding with low heat input provides benefits like these:
A very low heat input might, however, negatively affect the impact toughness (t8/5 values below 3 seconds). Figure 2 indicates the recommended maximum heat input (Q) for Hardox.
The cooling time (t8/5) is the time that it takes for the weld to cool from 800° – 500°C (1472° – 932°F), and it represents the key element of the final microstructure in the weld. Recommended cooling times are often provided for structural steels in order to optimize the weld process for a certain requirement, such as meeting the minimum impact toughness. Recommended maximum cooling times for different Hardox grades are available in SSAB’s WeldCalc software. Contact your local SSAB sales representative to find out more about WeldCalc.
Before tack welding, it is important to maintain a root opening between base plates not exceeding 3 mm (1/8”); see Figure 3. Aim for as uniform a gap size along the joint as possible. Also, avoid weld start and weld stops in highly stressed areas. If possible, the start and stop procedures should be at least 50 –100 mm (2” – 4”) from a corner; see Figure 3. When welding to the edge of plates, a runoff weld tab would be beneficial.
Due to a relatively low carbon equivalent, Hardox resists hydrogen cracking better than other wear-resistant steels.
Minimize the risk of hydrogen cracking by following these recommendations:
It is essential to follow the recommended minimum preheat temperature as well as the procedure for obtaining and measuring the temperature in and around the joint in order to avoid hydrogen cracking.
Influence of alloying elements on the selections of preheat and interpass temperatures
A unique combination of alloying elements optimizes the mechanical properties of Hardox. This combination governs preheat and interpass temperatures of Hardox steel during welding, and can be used to calculate the carbon equivalent. Carbon equivalent is usually expressed as CEV or CET according to the formulas below.
The alloying elements are specified in the mill certificate of the plate and are stated in weight percentages in these two formulas. A higher carbon equivalent usually requires a higher preheat and interpass temperature. The typical carbon equivalent for Hardox is stated in in SSAB’s product data sheets at www.ssab.com under Products>Brands>Hardox.
Minimum recommended preheat and maximum interpass temperatures during welding are given in Tables 5a, 5b and 6. Unless otherwise stated, these values are applicable for welding with unalloyed and low-alloyed welding consumables.
The interpass temperature shown in Table 6 is the maximum recommended temperature in the joint (on top of the weld metal) or immediately adjacent to the joint (start position), just before start of next weld pass.
The minimum recommended preheat and maximum interpass temperatures shown in Tables 5 and 6 are not affected at heat inputs higher than 1.7 kJ/mm (43.2 kJ/inch). The information is based on the assumption that the welded joint is allowed to air cool to ambient temperature. Note that these recommendations also apply to tack welds and root runs. In general, each of the tack welds should be at least 50 mm (2”) long. For joints with plate thicknesses of up to 8 mm (0.31”), shorter tack lengths may be used. The distance between tack welds can be varied as required.
The required preheat temperature can be achieved in several ways. Electric preheater elements (Figure 5) around the prepared joint are often best, since uniform heating of the area can be obtained. The temperature should be monitored by using, for example, a contact thermometer. We suggest that you measure the recommended preheating temperature on the opposite side of the heating operation; see Figure 6.
A minimum waiting time of 2 min/25 mm (2 min/ 1 inch) thickness should be conducted before measuring the preheating temperature. The minimum preheating temperature should be obtained in an area of 75 + 75 mm (3”+ 3”) around the intended weld joint; see Figure 6.
If the weld joint is located in an area with the expectation of high wear, you can employ hardfacing with special consumables to increase the wear resistance of the weld metal. Both the instructions for joining and hardfacing for Hardox should be followed. Some consumables for hardfacing require a very high preheat temperature that may exceed the maximum recommended interpass temperature for Hardox steel. It is worth noting that using a preheat temperature above the maximum recommended interpass temperature for Hardox steel may reduce the hardness of the base plate and result in deterioration of wear resistance of the preheated area.
Minimum and maximum preheat temperatures are the same as for conventional types of welding; see Tables 5a and 5b. See Figure 7 for the definition of single plate thickness for hard facing situations.
It is beneficial to weld a buffer layer with extra high toughness between the ordinary welded joint or plate and the hard facing. The choice of consumables for the buffer layer should follow the welding recommendations for Hardox wear plate. Stainless steel consumables in accordance with AWS 307 and AWS 309 should preferably be used for the buffer layer; see Figure 8.
The amount of distortion during and after welding is related to the base plate thickness and welding procedure. Distortion becomes more obvious in thinner gauges, while heavy deformation or even burn-through can cause problems and can compromise the whole structure.
Minimize the amount of distortion during welding by following these recommendations:
You can weld directly on Hardox primer thanks to its low zinc content. The primer can be easily brushed or ground away in the area around the joint; see Figure 13. Removing primer prior to welding can be beneficial, since it can minimize the porosity in the weld and can facilitate out-of-position welding. If primer remains on the weld surface, then the subsurface and surface porosity of the weld may be slightly higher. FCAW with basic flux offers the lowest porosity. It is important to maintain good ventilation in all welding processes to avoid the harmful effect the primer could have on the welder and surroundings.
Hardox HiTuf can be stress relieved by post-weld heat treatment, although this is seldom necessary. Other Hardox steels should not use this method for stress relieving, since this may impair the mechanical properties. For more information, consult the Welding Handbook from SSAB. You can order it at www.ssab.com.