**2. Material and Methodology**

Examinations were carried-out on Hardox Extreme steel sheets in an as-delivered condition, which was 1000 mm long and 10 mm thick. Welded joints were made by submerged arc welding *SAW* (121), while considering the welding materials dedicated for low-alloyed high-strength steels. Selected properties of the used welding materials are given in Table 4.


**Table 4.** Selected properties of welding materials used for Hardox Extreme joints [28].

*R*p0.2—yield strength, *R*m—ultimate tensile strength, *A*4—percentage elongation after fracture for proportional specimens with the original gauge length *L*<sup>0</sup> equal to 4 times diameter, *KCV*−40—Charpy V-notch toughness at −40 ◦C.

Joints were made using an automatic welding machine ESAB A2 Mini Trac with the power source ESAB LAE 800. The Hardox sheets were joined with a both-sides weld (Figure 2) using the following parameters guaranteeing correct penetration:


After welding, test specimens were cut-out in form of cuboids by means of a high-energy abrasive water stream (general specimen geometry) and through electroerosion (V-notch geometry). Next, a part of the specimens was subjected to heat treatment in laboratory conditions by being quenched in oil and tempering. It should be stressed that, with regard to the posed cognitive goals and available technical measures, the heat treatment operations were carried-out on whole specimens, i.e., for both heat-affected zone and base material. Before quenching, the specimens were additionally subjected to normalizing. All the thermal operations were carried-out in gas-tight chamber furnaces Czylok FCF 12SHM/R in a protective atmosphere of 99.95% argon. Quenching was carried-out in quenching oil Durixol W72 with a kinematic viscosity of 21 mm2/s, heated-up to 50 <sup>±</sup> <sup>5</sup> ◦C. Detailed characteristics of the specimens and heat treatment parameters are given in Table 5.

Chemical analyses were carried-out spectrally by means of a glow discharge spectrometer Leco GDS-500A, using the following parameters: *U* = 1250 V, *I* = 45 mA, 99.999% argon. The results were given as averages of at least five measurements.

Observations of macro- and micro-structures were performed using a multifunctional stereoscopic microscope Nikon AZ100 and a light microscope Nikon Eclipse MA200 coupled with a digital camera Nikon DS-Fi2. Images were recorded and analyzed using software NIS Elements.

Rockwell hardness (HRC/HRA) measurements were taken with a universal hardness tester Zwick/Roell ZHU 187.5 at 1500/600 kgf according to EN ISO 6508-1:2016-10. Measurements were taken on the specimens after microstructure examinations, within the base material (Hardox Extreme sheets) and in the zones subjected to structural analysis (lines A and B marked in Figure 2a).

**Figure 2.** General layout of a Hardox Extreme welded joint: (**a**) cross-section view; (**b**) view from the face of the weld. 1,2—individual welds acc. to their execution order; A,B—lines of hardness measurements; BM—base material; X,Y,Z—places of chemical analyses; KCV—way of cutting out specimens for impact tests—dimensions of the specimen after V-notching: 8.5 × 10 × 55 mm; UTS—way of cutting out specimens for ultimate tensile strength—dimensions of the cuboidal specimens with preset gauge length of L0 = 25 mm: 10 × 10 mm.

Mechanical tests were carried-out at ambient temperature according to EN ISO 6892-1:2016-09, using a testing machine Instron 5982, on cuboidal specimens with preset gauge length of *L*<sup>0</sup> = 25 mm (Figure 2b). The tensile tests were carried-out under controlled force to ensure a uniform strain rate for the specimens until their failure. Next, tensile strength (*R*m) and reduction of area at failure (*Z*) were determined.

Impact tests of the welded joints were carried-out using a pendulum Charpy tester Zwick/Roell RPK300 with initial energy 300 J, according to EN ISO 148-1:2017-02. The V-notch specimens used in the tests were cut from the entire butt joints (according to EN ISO 9016:2011) and included fusion zones of the analyzed welded joints in the conditions directly after welding and after heat treatment operations (Figure 2a). The tests were carried-out at +20 ◦C and −40 ◦C. Observations of fracture surfaces were performed with a stereoscopic microscope and a scanning electron microscope JEOL JSM-6610A. The SEM observations were performed at an accelerating voltage of 20 kV, in material contrast to what occurs when using SE detectors.

#### **3. Results**

Basic parameters of heat treatment operations carried-out on the examined welded joints are given in Table 5 and Figures 3 and 4, together with results of mechanical and impact tests of the joints in the conditions directly after welding and after heat treatment operations.

The performed heat treatment operations were aimed at obtaining a microstructure and mechanical properties in the entire welded joints that are similar to those of the base material. As such, the welded joints were volumetrically quenched in oil bath and tempered (stress relieved). The austenitizing temperature before quenching was established on the grounds of real chemical compositions of both the base material (Table 3) and weld metal, while considering complete cross-section of the welded joint (Table 6). Establishing the tempering temperature at 100 ◦C resulted from the fact that exposure of Hardox Extreme to temperatures over 125 ◦C causes decomposition of martensitic structure and significant decrease of hardness. This results also in lower indices from static tensile test and can lead to lower abrasive-wear resistance.


**Table 5.** Heat treatment parameters and selected mechanical properties of Hardox Extreme welded joint:

*R*m—ultimate tensile strength, *Z*—percentage reduction of area, *KCV*+20—charpy V-notch toughness at room temperature, *KCV*−40—charpy V-notch toughness at −40 ◦C.

**Figure 3.** Percentage reduction of area *Z* and toughness *KCV* of tested samples based on data from Table 5. AW—condition after welding, HT—condition after heat treatment.

**Figure 4.** Ultimate tensile strength *R*m of tested samples based on data from Table 5.




**Table 6.** *Cont.*

*CEV* [%] = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15; *CET* [%] = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40.3.1. X,Y,Z—places of chemical analyses, marked in Figure 2, *CEV*—carbon equivalent according to International. Institute of Welding, *CET*—carbon equivalent according to SS-EN 1011-2.

## *3.1. Mechanical Properties*

Average tensile strength of Hardox Extreme welded joints in the condition after welding reached *R*<sup>m</sup> = 1278 MPa (Table 5 and Figure 4), with maintained moderate plastic properties defined by relative area reduction *Z* = 17.4% (Table 5 and Figure 3). Even if the obtained strength is very high in comparison to that of constructional steels, it only provides ca. 64% of the value for the base material (assuming minimum tensile strength of Hardox Extreme equal to that of Hardox 600, i.e., 2000 MPa). Here, a significant scatter of the obtained values should be indicated. Even if the test joints were made on an automated welding station with significant lengths of run-off welds of over 150 mm, the results were characterized by rather low repeatability. In the author's opinion, such a behavior of these materials after welding is caused by uncontrolled structural changes occurring in the heat-affected zone, which is the probable cause of the unpredictable behavior of these steels in industrial conditions. This observation can be related mostly to plastic properties of the weld, which, to some extent, is also confirmed by relatively low, widely scattered impact strength values (especially at negative testing temperatures). Average impact strength at both testing temperatures of 17–18 J/cm<sup>2</sup> clearly shows the necessity to classify the obtained welded joints as being susceptible to brittle cracking.

As a result of the applied thermal treatments, all the considered mechanical properties of Hardox Extreme welded joints increased significantly (Table 5). Average tensile strength amounted to 1831 MPa, which makes nearly 90% of the assumed strength of the base material. In addition to very high strength indices, the plastic properties increased. The average value of relative area reduction at failure was *Z* = 29.1%, and impact energy occurred at an ambient temperature amounting to 27 J/cm2, allowing the supposition that ductile fracture provides a significant part of the entire fracture area.
