## 1 Introduction  
Non-destructive testing finds wide application both in industrial and laboratory settings. Among the most commonly used non-destructive methods, such as [1; 2; 3; 4] X-ray radiography, infrared, or electromagnetic applications, ultrasonic testing holds significant importance. The application of non-destructive methods can be very wide and can also include the testing of modern structures [5]. Ultrasonic testing utilizes the phenomena of refraction and reflection of ultrasonic waves, which are waves with frequencies above 16 kHz. Such testing is practically applied in every industry. The simplest ultrasonic technique involves measuring the thickness of elements accessible from one side, such as pipelines, tanks, boilers, and containers [6; 7]. As demonstrated in [8], it is possible to measure the thickness of elements made from various materials, such as aluminum, brass, polyester, and polyethylene, using the ultrasonic method. More complex examinations involve assessing the condition, detecting flaws occurring during production or operation, including cracks, shrinkage cavities, meltings, cross-forgings, or lack of fusion, assessment of joints, and the condition of components made in additive technology [9; 10; 11; 12]. These are standardized tests. Research in the field of ultrasonic testing encompasses various aspects, including the evaluation of properties of welded joints [13], the quality assessment of welded [14] and adhesive bonds [15; 16], as well as the evaluation of acoustic properties of materials produced using additive manufacturing technology [17]. These studies frequently encompass defect detection in different materials, not only in conventional ones like steel. In the study [18], the authors investigated the ultrasonic testing of two-phase materials. A more complex microstructure model within the bounds of double scattering response (DSR) was used. The new model was employed to calculate the corresponding grain noise limits, which were then used to define a time-dependent threshold for ultrasonic tests. A Ti-6 Al-4V sample with flat-bottom holes was manufactured for ultrasonic measurements to verify the measurement sensitivity for sub-surface defects. Ultrasonic flaw detection methods are relatively well-known, there has been work in which the effect of surface roughness, on the size of detectable defects was determined, but in a very limited range (low frequencies and low roughness) [19]. Both conducted studies and the available literature confirm that several factors can influence the quality and results of ultrasonic testing. Some of these factors are well understood and described in standards, for example, the adaptation of the ultrasonic transducer head to the shape of the tested element, as stated in ISO 5817:2014 [20]. One area that has not been fully explored is the influence of material properties and surface condition on the results of ultrasonic testing. Researchers have investigated the effect of hardness on ultrasonic wave velocity [21]. The authors demonstrated that the transmission, refraction, diffraction, and scattering characteristics of ultrasonic waves in the material depend on grain boundary features. They examined C1045 steels with varying hardness and established relationships between grain size, ultrasonic velocity, attenuation, and material hardness using two ultrasonic sources. The experimental results showed that a smaller average grain size and higher hardness can be achieved at higher tempering temperatures. Higher material hardness leads to faster acoustic velocities, and the scattering effect is more evident for higher transducer frequencies. The results of the experiment demonstrate that an alternative non-destructive testing method can evaluate the hardening process in industry. Similar studies [22] have used ultrasonic methods to examine hardness and property changes in welded joints.