Morphology of Welding Fume Derived from Stainless Steel Arc Welding ^†

1. Introduction

Fume morphology is the science of the structures, forms, shapes, fractions, and chemical composition of particles. It is a factor that determines their interactions with lung epithelial cells and, consequently, the effectiveness of their deposition in various regions of the lung [1].

During welding and allied processes, welding dust (a two-phase condensing aerosol) is emitted into the work environment, comprising a mixture of solid particles (fume) and gases [2]. Solid particles are formed as a result of the condensation and oxidation of metal vapors. The mechanism of fume formation is presented in Figure 1.

Metallurgical processes occurring during welding have a significant impact on the formation of welding fumes. These processes can be divided into three groups [4,5,6,7,8]:

(1)

Physical phenomena and chemical reactions occurring in the area of the electric arc on the contact surface of molten metal drops and the gas atmosphere of the arc;

(2)

Chemical reactions occurring in the weld pool and between the pool and the protective atmosphere;

(3)

Phenomena occurring in the heat-affected zone.

Welding dust-created fumes have been classified by the International Agency for Research on Cancer (IARC) as a group of agents with proven carcinogenic effects in humans [9].

The assessment of the risk related to exposure to welding fume emissions depends on the amount of fume generated, chemical composition, and morphology. The combined analysis of these factors determines the toxicity of the fume and its impact on the human body.

2. Research Results

The fume morphology analysis was carried out for samples generated during MIG welding of corrosion-resistant steels of grades 1.4301 and 1.4828. Grade 1.4301 (X5CrNi18-10) is an austenitic stainless steel with good corrosion resistance and ductility. This grade is resistant to most oxidizing acids, foodstuffs, sterilizing solutions, most organic chemicals and dyes, and inorganic chemicals [10]. The steel is also characterized by good weldability. It is used in the food, processing, dyeing, and construction industries, among others [10,11]. Steel grade 1.4828 (X15CrNiSi20-12) belongs to the group of austenitic heat-resistant steels and is widely used in industry. Its maximum operating temperature is 1000 °C, and it is characterized by good weldability and very good resistance to corrosion and oxidation. It can also be used in sulfate environments at temperatures exceeding 850 °C. This grade is mainly used in the production of industrial furnaces and heating elements, in the production of annealing equipment, in aerospace engineering, and in the automotive industry (e.g., in exhaust systems) [12,13].

The analysis of the structure (shape and dimensions) of welding fume particles was carried out using the following analytical methods:

• Laser diffraction;
• High-resolution scanning electron microscopy (HR-SEM).

2.1. Laser Particle Size Analysis

To determine the particle size distribution, the laser diffraction technique is applied. It uses the phenomenon of light scattering on particles. Light hitting an object is scattered, and the angle of deflection of the wave is strictly dependent on the size of the particle (the smaller the particle, the larger the scattering angle) [14]. The spectrum of scattered light is analyzed and the particle size distribution is obtained. The size distribution of fume particles deriving from arc welding of 1.4828 steel is shown in Figure 2.

In fume deriving from arc welding of the tested steel grades, 1.4301 and 1.4828, it was shown that the most numerous group included particles in the size range of 10–20 µm. In fume samples, the volume share of this fraction was over 31%. In fume samples, the second highest volume fraction, ranging from 21.65 to 22.77%, was found in the range with particle sizes of 20–30 µm. In turn, the volume fraction of particles from the 0–10 µm fraction ranged from 15.7 to 17.7%.

The analysis of the results showed that fume particles smaller than 20 µm accounted for nearly 50% of the total sample, and particles smaller than 30 µm accounted for more than 2/3 of the sample.

In all tested fume samples, nearly 99% were particles whose size did not exceed 100 µm. The volume fraction of particles larger than 100 µm did not exceed 1%.

The results obtained showed no influence of the grade of the base material on the particle size distribution. The differences for individual fractions depending on the steel grade did not exceed 2%.

In order to evaluate the risk related to fume emissions, the exact size distribution of particles belonging to the finest fraction (range from 0 to 10 µm) was determined. Fume particles belonging to the respirable fraction, those whose size does not exceed 3 µm, and the tracheal fraction, whose size ranged from 3 to 10 µm, are specified. The analysis of the results showed that over 3% of the fume from arc welding of corrosion-resistant steels belongs to the respirable fraction, i.e., particles reaching and penetrating the ciliated respiratory tract, and 12–14% to the tracheal fraction—particles penetrating outside the larynx. The tests were carried out on fume samples collected on filters, where, as a result of increased mobility due to high temperature, the particles create larger clusters.

2.2. Research Results Obtained Using High-Resolution Scanning Electron Microscopy

Welding fume samples were examined using a high-resolution MIRA 3 scanning electron microscope from TESCAN. It allows for research at magnification up to 250,000×. The observations confirm the information obtained during the literature review and analyzes performed using a digital microscope. Fume occurs in the form of single particles with an elongated or spherical shape, or in the form of chains or agglomerates (Figure 3).

Measurements of the size of fume particles were carried out; the results are shown in Figure 4. The morphology of the particles and their chemical composition were analyzed using the EDS method. The fume particles shown in Figure 4a have a diameter of 2.4–2.5 µm and contain mainly iron. At 25,000× magnification, it was possible to determine fume particles with dimensions of 0.3–0.5 µm (Figure 4b).

3. Summary

The results of the fume particle size distribution and the analysis of the shape and chemical composition using HR SEM with EDS in connection with the determination of the fume emission rate enable us to obtain an overall assessment of the health risk associated with welding fume. Such assessment is particularly important during the welding processes of corrosion-resistant steels due to the presence of chromium and nickel compounds in the fume, which are classified as substances with proven carcinogenic effects in humans (Group 1 according to IARC guidelines). It was found that 15–17% of particles deriving from arc welding belong to the respirable and tracheal fractions, which are the most harmful due to their penetration beyond the larynx.