*Proceedings* **Coordinate Measuring Machine Probes E**ff**ect during Inner Thread Position Measurement** †

**Ferencz Peti 1,\* and Petru Serban <sup>2</sup>**


Published: 28 December 2020

**Abstract:** Starting from the idea of improving Coordinate Measuring Machines' (CMM) measurement strategy for inner thread locations, we developed a new method which increases the accuracy of measurements and takes us closer to the pitch diameter. This article will analyze this new method by testing different touching probes configurations for different thread sizes. The objective is to identify the best probe configuration to be used in the measurements of different inner thread sizes.

**Keywords:** CMM measurement strategy; thread true position measurement; increase measurement accuracy

## **1. Introduction**

The evolution of technology creates the need to improve the response during production processes; to do this, the process control must be faster and have increased accuracy. In this research, we are focused on thread position measurement strategy solutions, because even if the CMM software developer elaborate solutions for regulated and non-regulated surfaces, the small inner threads (ex. M3 to M12) strategy were left behind. The actual measurement strategies that are used to measure inner threads on the CMM are divided in two categories. In the first, we can use the inner cylinder CMM strategy; here, we can chose different types of measurement, like more circles on different levels or an elliptical rotation with a pitch indexation/revolution or tangent lines to indicate the minimum or maximum diameter. In the second method, we use special pins, which will auto-center on the inner thread. Each method has pro and cons, and the actual paperwork is focused on the first measurement strategy, where improvements were made. The second strategy's advantages are the accuracy and the repeatability, the disadvantages are that the pins are very expensive and, overtime, the auto-centering will decrease, and repeatability will decrease, which will give bad measurement results. The CMM measurement accuracy is composed of two important categories: correct measurement strategy and correct tool used [1]. To improve the measurement accuracy, we elaborate a method where the measurement tool will scan the inner thread. This method was developed to improve the measurement strategy for this type of measurement. The second approach is to identify the best tool that can be used for different thread dimensions.

## **2. Material and Methods**

The measurements were performed on a Hexagon Global Advantage 122,210 Coordinate Measurement Machine edition 2018, Accuracy: 2.1 um + L/333, Drive Unit: HH-A-T-5 + (SP25M + SM25-3 + SH25-3)\*equipped with a steel probe with an active side made of a ruby ball with different

diameters: 2, 3, 4 and 5 mm. Software of CMM is PC-Dmis 2019R1. Measurement speed was set to 4 mm/s.

For the thread profile evaluation, a MahrMahrsurfXC2 Contour measurement machine edition 2017 with CD120 Drive unit was used, equipped with probe PCV 350 ± 9 with a 25 um active side of the probe, with an accuracy of 0.35 um. Measurement speed was set to1 mm/s.

The method that we used to make this evaluation consisted of measuring the thread as close as possible to the pitch diameter; this can be done by evaluating the form of the inner surface of the thread and identifying the closest point to the pitch diameter. The measurement will begin from this point and will measure a cylinder with a revolution movement that follows the pitch of the thread; more details can be observed in Figure 1. In the actual research, we connected the measuring probe diameter and the measurement strategy to measure a specific thread size. The probe configuration and dimensions are standardized, and therefore is possible to measure metric threads between M4 and M12 without a wide variety of probes. The dimensions of a probe are formed from an active area and inactive areas; examples are presented in Figure 2, where we can observe the active area represented by (D), the diameter of the probe, and inactive areas like M2, the metric thread used to assemble the probe, L, which represents the length of the probe that must be introduced in the CMM software to can make automatic calibrations, EWL, which is the length of the active ball support which, in combination with S, which represents the diameter of the EWL, will have a big impact when we chose the measurement probe. The measurement is done taking into consideration that characteristic "S" of the probe will not be in contact with the thread; the probe must be in contact only with the active side when measuring the inner thread.

**Figure 1.** Evaluation steps to measure the inner thread.

**Figure 2.** Probe configuration and dimensions: (**a**) Probe characteristics; (**b**) Characteristic marked with yellow are inactive and with green are active. "Pitch diameter measurements of internal threads are more complicated than external threads [2] due to construction of internal thread which does not allow easy probing like external threads".

#### **3. Results and Discussion**

Measurements were made for two metric threads, M6 and M8; for this, we used probes with an active sphere diameter from ø 2 to ø 5 mm. The results are focused on the pitch diameter and the repeatability and accuracy of thread location. Having all this information we investigate several issue and opportunities to do it right and here are some quote's from our refererences: "The basis of the direct method of measuring the position of the threaded holes was that the probe was led directly into the threaded hole and then were scanned the certain number of points, from these points cylinder by least-squares method was created. After that, the position of the axis of the cylinder to a reference hole was determined" [3]. "The methods presented apply only to determining the position of a threaded hole. The threads themselves, along with the minor diameter, should be checked with a thread gage with one exception. If they are large enough for a styli to fit inside and scan the surface, there are techniques discussed elsewhere that will help. None of the methods below will be able to calculate the size reliably (except scanning may get a reasonably close minor diameter) unless the pitch surface of the thread is accessible to a stylus" [4]. "Another problem measuring threads is if the threads on the CAD don't match the physical part, you'll have trouble finding the thread. Our part that we intended to measure threads on had a different start and end than what the model shows. So when I picked points along the major on the CAD, the CMM probed those points on the minor on the part. Not good. So we had to keep using threaded gages. Do the threads still work? yes they do, CAD is just the reference and I hear threads are a little difficult to model. Also we have multiple machines making the same part and there is start and end variation from those machines. So you'll not be able to use the exact same program/routine across CNC machines. First thing you have to do is be able find the thread with the CMM" [5].

#### *3.1. Metric 6 Measurement Results*

#### 3.1.1. Diameter Analysis

All diameter measurements are presented in Table 1. The results indicate that probe diameter ø 2 mm is the closest to the pitch diameter. If we consider going below this diameter, we risk that the part will be in contact with the inactive side of the probe.


**Table 1.** Diameters of the thread resulted by measuring using 3 probes and 5 measurements.

#### 3.1.2. Location Analysis

We calculate the true position (TP) for the measurement so that we have a simple interpretation of the results. Table 2 presents the measurement related to the diameter of the probe. Measurements made with probe ø 2 indicate the best results, next is probe ø 3 and the worst is probe ø 4.

**Table 2.** TP of the thread resulted by measuring using 3 probes and 5 measurements.


#### *3.2. Metric 8 Measurement Results*

#### 3.2.1. Diameter Analysis

All diameter measurements are presented in Table 3. The results indicate that probe diameter ø 2 and ø 3 mm is the closest to the pitch diameter. If we consider going below to this diameter, we will risk being in contact with the inactive side of the probe.

**Table 3.** Diameters of the thread, found by measuring using 4 probes and 5 measurements.


## 3.2.2. Location Analysis

We calculate the true position (TP) for the measurement so that we have a simple interpretation of the results. Table 4 presents the measurement related to the diameter of the probe. Measurements made with probe ø 5 indicate the best results, next is probe ø 2 and ø 3 and the worst is probe ø 4.

**Table 4.** TP of the thread, found by measuring using 4 probes and 5 measurements.


#### **4. Conclusions**

The measurement results indicated in Tables 1 and 3 show us that a small probe diameter can be a good solution to be closer to the pitch diameter; these results can also be seen in Tables 2 and 4. In these tables we measure the true position, and the evaluation indicates the same results as in Tables 1 and 2: a small probe is more accurate for this measurement strategy than a large probe. All the results presented above are made for a part where the machining tool is new. In the serial production, the regular measurement strategy remains the same for a CMM program, but the part will modify continuously during the cycle time of the project: wear of the mold, wear of the gages, wear of the machining tools, setup of new molds, etc., will all increase the chance of measurement errors. Knowing this, we created the variable strategy model where the thread is evaluated every time, and here the configuration of the probe also has a big impact on the final inspection of the part measurement. If we choose a probe with a small diameter, than we risk to measuring with the inactive part of the probe, whereas if we choose a probe with a large diameter, we will not measure the desired location. The wear of the machining tools (see Figure 3) facilitates the bad segment of thread and this will decrease the measurement availability area. The material defects can also facilitate a wrong measurement interpretation; the big porosity inside the threads must be identified before the measurement.

**Figure 3.** Thread section profile for a new tool (**a**), a used tool (**b**) and a broken tool (**c**).

If we use the correct measurement strategy and the correct measurement tool, we can increase the amount of correct measurements too. The pitch diameter gives us the most accurate and the most correct inner thread location and the ax given by this measurement indicates a correct location of the thread. We can find a false location of the threads if we are not focused on measuring these characteristics.

**Acknowledgments:** This work was partially supported by the UMFST "George Emil Palade" Quality Engineering and Digital Manufacturing Research Center.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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## *Proceedings* **Achieving a Toothed Gear on Presses** †

## **Marius Tintelecan 1,\* , Dana-Adriana Ilut,iu-Varvara 1, Oscar Rodriguez-Alabanda <sup>2</sup> , Ioana Monica Sas-Boca 1, Ionut, Marian <sup>1</sup> and Aristides Santana Martinez Gustavo <sup>3</sup>**


Published: 28 December 2020

**Abstract:** This paper presents a device that, in final connection with presses, allows toothed gear (with a crown similar to a toothed wheel with right teeth) to be achieved by hot deformation starting from a cylindrical steel bar. For this, finite element simulations were performed in Forge software. The proposed device has 23 rollers, so for the simulation process, a slice representing a part (the 23rd) of a circle which simplifies the essential functionality of this device was taken into consideration.

**Keywords:** toothed gear; hot deformation; presses; deformation roller; fibrous structure

## **1. Introduction**

From the start of this research problem, it should be noted that the semi-finished product was obtained through a plastic forming process (pressing) by an equipment with 23 forming rollers [1]. The symmetry axis of the set is collinear to the symmetry axis of the final toothed gear, so we subjected a slice which is the 46th part from 360◦, namely 7.8◦, in the simulations. Additionally, we tried to quantify the implications of operating the press punch on the designed and assembled toothed gear wheel.

A mesh network was built for each slice, and the corresponding plastic deformation equations were applied in each of them. Through this research methodology, we noticed how each simulated slice of 7.8◦ was affected overall, so we reached the necessary conclusions that led us to make the necessary changes in the whole assembly, and finally we saw how the whole set behaved.

The technical parameters of this plastic deformation process were deduced. It was proven that the technical feasibility for the manufacture of toothed gear wheels through this method is clearly superior to the classical method which involves milling, especially due to the significantly higher productivity of the proposed process and the material saving, in addition to the advantage of toothed gear wheels obtained with continuous fiber, who are much more resistant from a mechanical point of view.

## **2. The Pressing**

We started from the idea of making a toothed gear by pressing a cylindrical semi-finished product, made by drawing [2,3], through a roller device. In a specific case, we decided that the device should consist of 23 deformation rollers and a body (fixed) on which these rollers are assembled. The design was created using a CAD software: SolidWorks (see Figure 1). It must be noted that the device was made only for research, being intended for the realization of a gear with straight teeth having a certain number of teeth (23) and a certain circular pitch, respectively.

**Figure 1.** The principled sketch of the deformation mode with this device.

For practical, concrete cases, such a device can be built for each desired value of the number of teeth and the circular pitch "p" of the final toothed gear.

The toothed gear achieved device, built, has 23 deforming rollers, for simulation using the 46th part of a virtual circle, which has in the center the longitudinal axis of symmetry of the semi-finished product, thus being similar to a slice with peak angle of 7.8◦ as is shown in Figure 2.

**Figure 2.** The aspect of the "slice" subjected to simulation ((**a**)—longitudinal view, (**b**)—cross-sectional view, 1—the moving force, 1 —the direction of movement, 2—the semi-finished product, 3—the deforming rollers, 4—the body of device).

This simplification was used to speed up the analysis time of the simulation process. We want to mention that the simulation of the deformation process was simplified to a "slice" because the process is axisymmetric.

#### **3. The Device Description**

The previously proposed device was actually made to confirm the simulation results and its design and real assembly is shown in Figure 3.

**Figure 3.** The device for achieving of a toothed gear on presses: (**a**) the proposed version, (**b**) the built version; the forming rollers 3 and the body (fixed) of device 4. The notations are in correspondence with those from Figure 1.

Starting from a cylindrical semi-finished product (see Figure 4a), the device manages to make toothed gears by hot forming, similar to that shown in Figure 4b.

**Figure 4.** The initial semi-finished product (**a**); the final product (**b**).

In fact, the intermediate technical stages (viewed in cross section) are detailed in Figure 5.

**Figure 5.** Cross-sectional view of the processed semi-finished product: (**a**) the initial semi-finished product (green) and (**b**) the final product (red); the forming rollers are numbered as 1, 2, 3.

#### **4. Simulation**

The simulations were performed by finite element analysis (FEA), using FORGE software.

#### *4.1. The Dimension Assessment of the Initial Semi-Finished Product*

Basically, the diameter Ds of the semi-finished product and its height Hs were determined. The mathematical calculation of the diameter Ds of the semi-finished product implies that the sum of the transverse areas of the portions displaced by the action of the forming rollers must be equal to the sum of the transverse areas of the tooth tips formed by the flow of the material [4] (see Figure 6).

**Figure 6.** (**a**) Main view of the semi-finished product deformation and (**b**) the previous view enlarged with the dimensions that appear in the nomenclature.

*Proceedings* **2020**, *63*, 57

We estimated the area affected by the forming rollers, corresponding to the area displaced by their action (I) and the area of the tooth tip, formed by the flow of the material (II), respectively, with two trapezoids [5] whose surfaces are:

$$\mathbf{S}\_{\rm I} = \frac{\mathbf{f}\_{\rm d} + \mathbf{s}\_{\rm d}}{2} \cdot (\mathbf{D}\_{\rm s} - \mathbf{D}\_{\rm f}) \tag{1}$$

$$\mathbf{S}\_{\rm II} = \frac{\mathbf{v}\_{\rm d} + \mathbf{e}\_{\rm d}}{2} \cdot (\mathbf{D}\_{\rm V} - \mathbf{D}\_{\rm s}) \tag{2}$$

(fd + sd) <sup>2</sup> ·(Ds <sup>−</sup> Df) <sup>=</sup> (vd <sup>+</sup> ed) <sup>2</sup> ·(Dv <sup>−</sup> Ds) fd·Ds + sd·Ds <sup>2</sup> <sup>−</sup> fd·Df <sup>+</sup> sd·Df <sup>2</sup> <sup>=</sup> vd·Dv <sup>+</sup> ed·Dv <sup>2</sup> <sup>−</sup> vd·Ds <sup>+</sup> ed·Ds 2 fd·Ds + sd·Ds <sup>2</sup> <sup>+</sup> vd·Ds <sup>+</sup> ed·Ds <sup>2</sup> <sup>=</sup> vd·Dv <sup>+</sup> ed·Dv <sup>2</sup> <sup>+</sup> fd·Df + sd·Df 2 so, (fd + vd + sd + ed)· Ds <sup>2</sup> <sup>=</sup> (vd <sup>+</sup> ed)· Dv <sup>2</sup> <sup>+</sup> (fd <sup>+</sup> sd)· Df <sup>2</sup> (3)

The approximation of vd ≈ fd and sd ≈ ed results in:

$$\mathbf{p} \left( \mathbf{v\_d} + \mathbf{s\_d} \right) \cdot \mathbf{D\_s} = \left( \mathbf{v\_d} + \mathbf{s\_d} \right) \cdot \frac{\mathbf{D\_v}}{2} + \left( \mathbf{v\_d} + \mathbf{s\_d} \right) \cdot \frac{\mathbf{D\_f}}{2} \tag{4}$$

$$\mathbf{D}\_s = (\mathbf{D}\_\mathbf{v} + \mathbf{D}\_\mathbf{f})/2 \tag{5}$$


To correct the approximations from these calculations, FEA simulations were performed determining the von Mises stress and deducing the optimum dimensions of the initial semi-finished product: diameter, Ds (Figure 7), and height, Hs (Figure 8). The dimensions Ds and Hs are not specified as a value and are only relevant in correspondence with the dimensions of the device object of study. These dimensions were established so as to achieve the maximum degree of filling of the toothed gear by the flow of the material and a certain behavior in the process of its deformation.

**Figure 7.** The deduction of the diameter Ds of the initial semi-finished product by finite element analysis (FEA) simulation.

**Figure 8.** The deduction of the height Hs of the initial semi-finished product by FEA simulation.

## *4.2. The Von Mises Stress Determination*

In the second phase, with these correct dimensions, a mesh network was applied both for the semi-finished product and for the analyzed device. Thus, the final values of von Mises stress were deduced. Because both the semi-finished product and the analyzed device were involved in the simulation at this moment, a view was achieved in its longitudinal section, focused on the von Mises stress that appeared at the deformation of the semi-finished product [6]. Figure 9 shows 5 figures made at various deformation moments: (**a**) corresponds to the deformation start time t = 0 (initial deformation moment), (**b**) corresponds to the moment t/5, (**c**) moment (2/5)·t, (**d**) moment (3/5)·t, (**d**) moment (4/5)·t and (**e**) the final moment (t = t).

**Figure 9.** The von Mises stress involved in the whole process of plastic deformation of the semi-finished product (the duration of the deformation was symbolized by "t").

## *4.3. The Semi-Finished Product Deformations*

This simulation is similar as aspect to the previous one, practically focusing on the amplitude of the deformations of the semi-finished product along the entire time interval (t) of its deformation, as shown in Figure 10.

**Figure 10.** The deformations involved in the whole process of plastic deformation of the semi-finished product.

## *4.4. The Flow of the Semi-Finished Material*

Mesh network was no longer viewed. The whole system device-part was cut from the entire longitudinal section with the aim of analyzing only the evolution of the flow of the part material, as shown in Figure 11.

**Figure 11.** The flow of the semi-finished material, in the process of its plastic deformation.

## **5. Conclusions**

In this research, it was proven that toothed gears with a certain circular pitch and number of teeth can be achieved by this method [7] of plastic deformation, and as with any process of plastic deformation, the fibrous structure being continuous, it results in a more resistant final product, able to respond more efficiently to various mechanical stresses [8]. During the practical hours of simulation, it was also observed that the flow of the material in the tooth formation imposes a certain regime of deformation speed. It was also observed that in the case of obtaining steel toothed gears, the respective deformation process will be performed hot, requiring only the heating of the cylindrical surface of the initial semi-finished product.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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## *Article* **Reuse of the Steel Mill Scale for Sustainable Industrial Applications** †

## **Dana-Adriana Ilut,iu-Varvara \*, Marius Tintelecan , Claudiu Aciu and Ioana-Monica Sas-Boca**

Department of Building Services Engineering, Faculty of Building Services Engineering, Technical University of Cluj-Napoca, 28 Memorandumului Street, 400114 Cluj-Napoca, Romania;

mariust@ipm.utcluj.ro (M.T.); claudiu.aciu@ccm.utcluj.ro (C.A.); monica.sas.boca@ipm.utcluj.ro (I.-M.S.-B.)

**\*** Correspondence: dana.varvara@gmail.com or dana.adriana.varvara@insta.utcluj.ro

† Presented at the 14th International Conference INTER-ENG 2020 Interdisciplinarity in Engineering, Mures, , Romania, 8–9 October 2020.

Published: 11 December 2020

**Abstract:** The purpose of our paper is to assess the reuse potential of the steel mill scale for sustainable industrial applications. We have presented the experimental procedures for chemical and mineralogical characterizations. According to the results of the elementary chemical analysis, the steel mill scale contains the following predominant chemical elements: iron, aluminum, silicon, and magnesium. Due to its high iron content, the steel mill scale can be reused as a source of raw material in the sustainable steelmaking industry. The mineralogical phases identified in the steel mill scale are: wüstite (FeO), hematite (Fe2O3), magnetite (Fe3O4), silica (quartz) (SiO2), magnesioferitte (MgFe2O4), and aluminum oxide (corundum) (Al2O3). Silica, alumina, and hematite are the main compounds of the cement and contribute to the formation of the: dicalcium silicate (2CaO·SiO2), tricalcium silicate (3CaO·SiO2), tricalcium aluminate (3CaO·Al2O3), and tetra—calcium aluminoferrite (4CaO·Al2O3·Fe2O3). The results of the paper are promising and encourage the future research for establishing the optimal percentage for the reuse of the steel mill scale in the composition of concrete.

**Keywords:** reuse; sustainable industry; steel mill scale; mineralogical characterization; industrial wastes; cement; steelmaking; alloying elements

## **1. Introduction**

Sustainable development was introduced in a widespread way by the Brundtland Commission, which defined it as development that "meets the needs of the present without compromising the ability of future generations to meet their own needs" [1]. Sustainability has been applied to many fields, including engineering, manufacturing, and design. Manufacturers are becoming increasingly concerned about the issue of sustainability. For instance, recognition of the relationship between manufacturing operations and the natural environment has become an important factor in the decision making among industrial societies [2].

Sustainable manufacturing focuses on both how the product is made as well as the product's attributes. This includes the inputs, the manufacturing processes, and the product's design. Sustainable manufacturing includes things such as making products using less energy and materials, producing less waste, and using fewer hazardous materials as well as products that have greener attributes such as recyclability or lower energy use [3].

The European steel industry generates an estimated 500,000 tones/yr of oily sludge and mill scales. More than 30% of this total is not valorised. The steelmaking by-products such as dust and mill scale are currently produced in large quantities and represent a potential of almost 5 million tons in the world [4,5].

Mill scale is a steelmaking by-product from the rolling mill in the steel hot rolling process. Mill scale can be considered a valuable metallurgical raw material for iron making, steelmaking, and construction industries because it contains valuable metallic minerals [6–8].

The chemical and mineralogical characterizations of steel mill scale play a key role for their reuse in sustainable industrial applications.

The aim of this paper is the assessment of the reuse potential of the steel mill scale for sustainable application, both in the steelmaking and building materials industries.

The objectives of the paper are:


## **2. Materials and Methods**

The steel mill scale sample was taken from a metallurgical plant (Salaj County, Romania). The mill scale comes from the rolling process of the steel pipes. In order to identify the possibilities of reusing the mill scale, for sustainable industrial applications, the sample was subjected to chemical and mineralogical characterization. The chemical elements from the steel mill scale sample were determined using inductively coupled plasma. The mineralogical characterization of the mill scale sample was performed with the help of an X-ray diffractometer, Brucker Advance D8 type (Germany). The identification of the mineralogical phases was made with Match software from Crystal Impact. This software uses the PDF database from International Centre for Diffraction Data.

## **3. Results and Discussions**

Table 1 shows the major and minor chemical elements contained in the steel mill scale.


**Table 1.** Major and minor chemical elements contained in the steel mill scale.

According to the results of the chemical analysis, the major elements in the steel mill scale composition are iron, aluminum, silicon, magnesium, and manganese. The main constituent of the mill scale is iron with 76.8%. Due to its high iron content, the steel mill scale can be reused as a source of raw material in the sustainable steelmaking industry. According to the reference [9], the reuse of iron, from the steel mill scale, as a raw or auxiliary material to the steelmaking, leads to natural resources conservation. The minor elements contained in the steel mill scale are chromium, nickel, molybdenum, copper, zinc, vanadium, cadmium, calcium, and, arsenic, etc. According to the references [9–11], the chromium, nickel, molybdenum, manganese, and vanadium can be reused as alloying elements to the stainless steelmaking in the electric arc furnace.

Figure 1 shows the diffractogram of the steel mill scale.

**Figure 1.** Diffractogram of the steel mill scale.

Table 2 shows the chemical formulas and powder diffraction files of the mineralogical phases identified in the steel mill scale sample, by X-ray diffraction.

**Table 2.** Mineralogical phases identified in the steel mill scale by X-ray Diffraction (XRD).


According to the data presented in the Figure 1 and Table 2, the mineralogical phases identified in the steel mill scale are: wüstite (FeO), hematite (Fe2O3), magnetite (Fe3O4), silica (quartz) (SiO2), magnesioferitte (MgFe2O4), and aluminum oxide (corundum) (Al2O3).

The results of the X-ray diffraction show that the mineralogical phases identified in the steel mill scale are also found in the mineralogical composition of the Portland cement. According to the references [12,13], the Portland cement consists mainly of lime (CaO), silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3). In conclusion, silica, alumina, and hematite are the main mineralogical phases both in steel mill scale and in the Portland cement.

Silica, alumina, and hematite are the main compounds of the cement and contribute to the formation of the dicalcium silicate (2CaO·SiO2), tricalcium silicate (3CaO·SiO2), tricalcium aluminate (3CaO·Al2O3), and tetracalcium aluminoferrite (4CaO·Al2O3·Fe2O3). According to the references [8,14], the steel mill scale can be reused in the cement and mortar compositions.

The mineralogical composition of the steel mill scale plays a key role in establishing the reuse domains.

#### **4. Conclusions**

The main constituent of the mill scale is iron with 76.8%. Due to its high iron content, the steel mill scale can be reused as a source of raw material in the sustainable steelmaking industry.

The mineralogical phases identified in the steel mill scale are: wüstite (FeO), hematite (Fe2O3), magnetite (Fe3O4), silica (SiO2), magnesioferitte (MgFe2O4), and alumina (Al2O3). Silica, alumina, and hematite are the main mineralogical phases both in steel mill scale and in the Portland cement. These mineralogical phases contribute to the formation of the: dicalcium silicate, tricalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. The results of the paper are promising and encourage the future research for establishing the optimal percentage for the reuse of the steel mill scale in the sustainable building materials.

**Author Contributions:** Conceptualization D.-A.I.-V., M.T., C.A. and I.-M.S.-B.; methodology D.-A.I.-V.; investigation D.-A.I.-V. and C.A.; writing—original draft preparation D.-A.I.-V., M.T., C.A. and I.-M.S.-B.; writing—review and editing D.-A.I.-V., M.T., C.A. and I.-M.S.-B.; visualization D.-A.I.-V.; supervision D.-A.I.-V.; project administration D.-A.I.-V.; funding acquisition D.-A.I.-V. All authors have read and agreed to the published version of the manuscript.

**Funding:** We acknowledge support by the Technical University of Cluj-Napoca.

**Acknowledgments:** This paper is written within the TUCN Internal Research Project Competition 2016 "Research concerning the characterization of the oily mill scale in order to identify an optimum method for reduction of the quantities of hazardous wastes landfilled", internal competition for Research/Development/Innovation—Project 16362/07.07.2016, C.I. type 1.1-T4, Technical University of Cluj-Napoca (2016). The Internal Research Project Competition is funded by the Technical University of Cluj-Napoca in order to support the internal accredited research structures.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
