The Metals Quality Analyzer MQA - Thermo Fisher …...The Metals Quality Analyzer MQA Process design of inclusion modification in cast steel using automated inclusion analysis Vintee

The Metals Quality Analyzer™ MQAProcess design of inclusion modification in cast steel using automated inclusion analysis

Vintee Singh, Simon Lekakh, and Kent D. Peaslee Department of Materials Science & EngineeringMissouri University of Science and Technology, Rolla, MO

Timothy J. DrakeApplication SpecialistAspex Corporation, Delmont, PA

World Headquarters5350 NE Dawson Creek DriveHillsboro, Oregon 97124 USA

Phone: +1 503 726 7500

AISTech 2009 Proceedings Paper. 1

Process Design of Inclusion Modification in Cast Steel using Automated Inclusion Analysis

Vintee Singh1, Simon Lekakh1, Timothy J. Drake2, and Kent D. Peaslee1a, 1Department of Materials Science & Engineering, Missouri University of Science and Technology, Rolla, MO 65409-0330, United

States 2Aspex Corporation, Delmont, PA

aCorresponding author: Ph: (573) 341-4714; Fax: (573) 341-6934; E-Mail: [email protected]

Key words: Inclusions, Steel foundry, Deoxidation, Automated analysis INTRODUCTION This paper studies the effects of changes in melting and ladle practices (deoxidation, slag, refractory types, etc.) in steel foundries on steel cleanliness. The effects of deoxidation and pouring practices on the size, type and number of inclusions were compared for steel foundries using induction and arc furnaces of capacity 1 to 20 tons. Samples collected from the furnace, ladle, and cast products were analyzed using an automated Aspex Metals Quality Analyzer. This study summarizes the optimum deoxidation and ladle treatment for inclusion reduction and modification in the smaller industrial furnaces used by foundries where the use of traditional ladle furnaces is impractical. DISCUSSION An increase in demand for higher quality steel is forcing steelmakers to ensure that their steel products meet more stringent ‘‘cleanliness’’ requirements. The mechanical properties for steel are affected by the volume fraction, size, distribution, composition, and morphology of inclusions. Fracture toughness of steel decreases when inclusions are present, especially in higher-strength lower-ductility alloys. 1 Hence, the exact determination of non-metallic inclusions is essential to the success of research aimed at increasing toughness of steel parts. In cast steel, non-metallic inclusions are the primary sites at which void nucleation occurs. The voids nucleated at the particle sites grow until they coalesce by impingement or by the process of void sheet coalescence.2 Void sheet coalescence requires fracture of the ligament between the voids created at the larger non-metallic inclusions. Figure 1 shows that inclusions cause voids, which will coalesce and induce fracture if they are larger than a critical value. Inclusions tend to be larger than other second phase particles in steels, such as those precipitated during heat treatment, ranging in diameter from about 0.1–10 μm or more. The characteristic inclusion volume fraction and the inclusion spacing have been shown to greatly influence the toughness of steel. Fracture toughness was determined to be indirectly proportional to the volume fraction of inclusions present in the sample.4 Sources of inclusions exist throughout the steel making process. Deoxidation products, such as alumina inclusions, constitute in many cases the majority of the indigenous inclusions.5 Oxide inclusions are generated by the reaction between the dissolved oxygen and the added deoxidant, such as aluminum. Alumina inclusions are dendritic when formed in a high oxygen environment or may result from the collision of smaller particles, as shown in Figure 2(a,b,c).6,7 Many of the inclusions existing in castings are also formed by reoxidation in which liquid steel, having "free" deoxidants (Al, Mn or Ca) dissolved in the melt, picks up oxygen from contact with the air during pouring and transportation through the gating system. In addition, inclusions can be formed in the casting by reaction of the liquid steel with water vaporizing from the molding sands and the eroding debris in the gating

system. Inclusions formed due to slag entrapment are usually spherical, as shown in Figure 2(d).7

a)

b)

Figure 1: a) Void nucleation on non-metallic inclusions and steel fracture2 and b) SEM of void nucleation and linking during deformation.3



Figure 2: Typical inclusions morphology in cast steel, with different composition and sources

Elements having high oxygen affinity are used as de-oxidizers of cast steel. Figure 3 shows that Al has the best deoxidation ability, followed by Ti, Zr, and Si.8 Aluminum is most commonly used for deoxidation of steel castings. Calcium treatment is used to reduce the harmful effects of Al2O3 inclusions. The addition of calcium promotes partial reduction of the Al2O3 inclusions, giving rise to the formation of liquid calcium aluminates with low melting point and spherical morphology, which can easily float out. Most of these liquid inclusions separate easily from the melt, and those not removed are less harmful to the mechanical properties of the final steel product. The reaction sequence followed is:

Al2O3 -> CA6 -> CA2 -> CA -> C12A7 where, C and A denote CaO and Al2O3, respectively.9 The CaO- Al2O3 phase diagram in Figure 4 shows the presence of CA2, CA, C12A7 in liquid state at steelmaking temperatures (~ 1650 oC).10 Ca treatment also modifies the intergranular fine MnS inclusions to globular CaS inclusions, which are less harmful to the mechanical properties due to the spherical shape.11 Figure 5 shows SEM and EDS images of how calcium treatment modifies MnS and alumina inclusions. Steel cleanliness can be indirectly quantified by measuring the total oxygen content in the steel. The total oxygen in the steel is the sum of the “free” oxygen (dissolved in the melt) and the oxygen combined as non-metallic inclusions. “Free” or “active” oxygen can be measured relatively easily using oxygen sensors. Because the variation of free oxygen is minimal, the total oxygen is a reasonable indirect measure of the total amount of oxide inclusions in the steel. Low total oxygen content decreases the probability of large oxide inclusions 12, as shown in Figure 6. Thus, total oxygen is a very important and common index of steel cleanliness.

Figure 3: Deoxidation results of the common deoxidizers8

Figure 4: CaO- Al2O3 binary phase diagram 10

MODIFYAlumina Ca Aluminates

MnS CaS Figure 5: SEM and EDS images of alumina and MnS modification on Ca-treatment, obtained through Aspex MQA.

Figure 6: Relationship between total oxygen and macro-inclusions content in steel 7

In the past, scanning electron microscopy (SEM) analysis combined with energy dispersive spectroscopy (EDS) has been used for inclusion characterization. However, manual SEM approaches are limited by the labor intensive nature of the analysis, the SEM operator must analyze



by manually imaging the individual inclusions. The equipment used for this research, the Metals Quality Analyzer (Aspex Corporation, Delmont, PA) provides a rapid and accurate method for determining the composition, size, number, spacing and distribution of inclusions present in steel samples. The MQA is an integrated SEM and EDS system and allows for automated characterization of all the inclusions (1 μm to 5 mm) in a microscopic specimen; including the volume fraction, size and shape, inclusion spacing and complete inclusion identification. With the use of this equipment, the possible number of inclusions analyzed could be increased dramatically from the tens to the thousands, while decreasing the total time involved from days to minutes. PROCEDURE Plant Trials: Plant trials were conducted at four foundries with different deoxidation practices in the furnace and ladle as summarized in Table I. Three foundries were induction furnace based (IF) and one foundry had electric arc furnaces (EAF). During the plant trials, the dissolved oxygen content was measured directly in the melt, using Celox oxygen probes and the Celox Lab Datacast-2000. Steel samples were collected from the furnace and the

ladle, before and after the addition of deoxidants. In addition, samples were cut from castings produced from the same melt. Microscopic specimens were prepared from these samples and a 10 mm2 area was analyzed in each specimen for inclusions using the Aspex MQA for automated inclusion analysis. The total oxygen content was measured from each sample using Leco TC-500. MQA Analysis:

In the Aspex MQA system, a focused electron beam is moved across the specimen in an array of fairly coarse steps, as shown in Figure 7. The mechanical fields are subdivided into smaller fields. For example, a 4X4 mm square sample field can be broken down into 16 fields of 1x1mm square that are defined electronically. That is, the beam is displaced by the scan system so that each 1mm area is treated as a separate field, reducing the number of time-consuming mechanical stage motions sixteen-fold. As the electron beam moves across each field, the brightness or intensity of the back scattered electron detector (BSED) signal is recorded and transferred to the computer memory as representing the brightness of a single pixel (Figure 7b). If the signal is bright enough to indicate that an inclusion is present at the position, the software initiates a particle-sizing sequence using a rotating chord algorithm. Once the coarse scanning (indicated by the dots) identifies an inclusion, the center is identified and chords are drawn with the beam on the inclusion to define the size and shape of the inclusion.

Table I. Melting and deoxidation practices at the four foundries

Charge Weight (lbs.) Furnace additions (in wt. %) Ladle additions (in wt. %)

Plant A (IF) 1000 - Added at tap:

Al (0.1%), FeTi (0.035% Ti), FeSiZr (0.04% Si, 0.03% Zr)

Plant B (IF) 1400 Al (0.08%) Added at tap: CaSi(0.08% Ca, 0.3% Si)

Plant C (Acid EAF) 40,000 FeMn+FeSi Block additions

Added at tap: Al (0.07%)

Wire fed in ladle CaSi (0.06% Ca, 0.10% Si)

Plant C (Basic EAF) 40,000 FeMn+FeSi Block additions

Added at tap: Al (0.068%)

Wire fed in ladle CaSi (0.04% Ca, 0.08% Si)

MS&T Foundry (IF) 100 Al (0.10%) Added at tap: CaSi(0.09% Ca, 0.17% Si)

a) b) c) Figure 7: Automated inclusion analysis including a) subdividing the image into fields, b) moving electron beam across field in an array and c) sizing and cataloging of particles detected by back scattering electrons and centering the beam on each particle and obtaining composition by x-ray spectroscopy13



This is a fast process because the instrument only spends time collecting detailed sizing data where inclusions are known to be present, rather than spending time capturing and analyzing vast numbers of essentially empty pixels. Subsequently a number of size and shape parameters are computed from the lengths of the chords.

After inclusions have been fully characterized for size and shape, an EDS spectrum is acquired to determine the elemental composition of each inclusion. While the system is programmed to identify the inclusions based on the library set of definitions determined by size and composition information during the run, the data for this study was evaluated offline by the Automated Feature Analysis Data Viewer software after the sample had been completely analyzed and the data stored. The inclusions are classified into various classes based on their composition as determined by user-defined rules. For example, an inclusion with Mn >= 30% and S >=20% is classified as a MnS inclusion. Each sample was analysed on the Aspex 3 to 5 times, in order to check the accuracy of the inclusion measurements within a single sample. In order to measure the average spacing between the inclusions in the specimens, a code was written in Visual Basic. This code first calculates the distance of an inclusion from each of the other inclusions and determines the distance to the closest neighbor. These distances are averaged over all inclusions to determine the average spacing between the inclusions in the specimen.

RESULTS AND DISCUSSION

Plant A In the trial at Plant A, a heat of medium-carbon steel (WBC) was produced in an induction furnace and tapped into a 1000 lb capacity ladle. Al, FeTi and FeSiZr were added as deoxidants in the ladle. Figure 8 compare the area fraction covered by inclusions and the total and dissolved oxygen, at the various stages of liquid processing, as measured by the MQA system. The area fraction represents the fraction of the area covered by inclusions. The area of alumina and TiO2 inclusions increased after the Al and FeTi additions in the ladle. The area of the oxide inclusions increased during the pour and also the casting had more inclusions than in the ladle. This indicates that there is significant reoxidation during pouring and the melt transport through the gating system. Also, there is insufficient time to float inclusions in the ladle. The dissolved oxygen dropped after deoxidation, resulting in the formation of a large number of oxide inclusions and an increase in the total oxygen. As the steel processing progressed, the total oxygen increased indicating reoxidation and a lack of inclusion flotation. Plant B In this plant trial, one induction furnace heat was followed from melting through deoxidation and pouring of a medium-carbon steel (8625 alloy) in a 1400 lb ladle. For deoxidation, Al was added in the furnace just before tap followed by a CaSi addition in the tap stream. Figure 9 shows the area fraction covered by inclusions and the dissolved and total oxygen for samples collected during various stages of the casting process. Figure 10 show the ternary chemical mapping for sulfides and oxides before and after Ca additions in the ladle. The region of calcium aluminates (CA) is circled on the ternary oxides diagram. After Al treatment in the furnace, there was an increase in alumina inclusions and the total oxygen. The composition and number of inclusions changed after the Ca treatment in the ladle with most of the alumina inclusions forming calcium aluminates (CA). Significant reoxidation was observed with all of the oxide inclusions increasing from the ladle through casting. MnS inclusions were first observed in the furnace and remained fairly constant through the ladle and casting.

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Figure 8: Comparison of (a) inclusion volume and b) dissolved and total oxygen measured in samples collected at various stages of the casting process (Plant A).

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Figure 9: Comparison of (a) inclusion area and (b) dissolved and total oxygen measured in samples collected at various stages of the casting process (Plant B).



Calcium

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b) Figure 10: a) Sulfides Mn-Ca-S and b) oxides (Mn+Si)-Ca-Al mapping for the casting process, before and after addition of Ca in the ladle (Plant B).

As seen in the ternary oxide and sulfide mappings, contrary to the Ca modification of the alumina inclusions, there was limited Ca modification of the MnS inclusions. Adding Ca in the form of CaSi ferroalloy during tap is inconsistent in its metallurgical effectiveness. CaSi ferroalloy was observed to float on the melt surface often flashing indicating vaporization of Ca followed by rapid combustion in air. Ca is highly volatile with a boiling point of 1500oC making it difficult to add to the steel without losing it to vaporization. Injection of calcium below the surface of the steel through wire or powder injection would be more effective as it suppresses Ca boiling because of the higher ferrostatic pressure.14

Plant C In this study, two EAF heats of medium-carbon steel were followed, with ladle of capacity 20 tons. The first one utilized an acid refractory and slag practice and the second one used a basic refractory and slag practice. Ladle treatment included preliminary deoxidation during tap followed by treatment with Ca wire in the ladle. During the heat, chemistry samples and dissolved oxygen readings were collected at the following locations:

1. Furnace after block 2. Ladle before calcium wire treatment 3. Ladle after calcium wire treatment 4. Ladle at mid-ladle pouring. 5. Sample from the final casting.

Figures 11 and 12 show the fraction of area covered by inclusions and the dissolved and total oxygen content for both acid and basic processes. In the acid process, the area of inclusions decreased after addition of the Ca due to the formation of calcium aluminates (CA). Also in this process, MnS inclusions were modified to CaS. In the basic process, many of the alumina and MnS inclusions were not successfully modified. Overall, the acid process had more inclusions than the basic process, which is supported by the higher levels of total oxygen.

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Figure 11: Inclusion area fraction in samples collected at various stages of the casting process for a) acid and (b) basic process (Plant C).

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Figure 12: Dissolved and total oxygen measured in samples collected at various stages of the casting process for (a) acid and (b) basic process (Plant C).



Figure 13 summarizes the inclusion composition for the acid process using a ternary mapping system for sulfides and oxides. After the Ca addition, the MnS inclusions decreased and replaced by CaS inclusions. This is a desired transformation due to the globular morphology of CaS. Also, a significant amount of CA formation was observed with a decrease in both MnO and Al2O3 inclusions. Figure 14 presents a binary phase MnS-CaS diagram, which shows that MnS and CaS could form a solid solution above 1150oC. This transformation of MnS to MnS-CaS solutions and relatively pure CaS was also observed in the Mn-Ca-S inclusion mapping.

Before After

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Treatment

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b)

Figure 13: a) Sulfides Mn-Ca-S and b) oxides (Mn+Si)-Ca-Al mapping for casting process, before and after addition of Ca in the ladle (Plant C, Acid process).

Figure 14: Binary MnS-CaS phase diagram14

Figure 15 shows the ternary chemical mapping of the sulfide and oxide inclusions for the basic process. Contrary to the acid practice, there was not much of a decrease in MnS and Al2O3 inclusions in the basic practice. Also, formation of the desired CaS or CA inclusions was not observed in the basic process. This can be explained as more CaSi wire was added to the ladle in the acid process (0.06 wt. %) as compared to the basic process (0.04 wt. %). The amount of Ca in the basic process was not sufficient to cause the modification of MnS and alumina inclusions.

Calcium


a)

Calcium


b) Figure 15: a) Sulfides Mn-Ca-S and b) oxides (Mn+Si)-Ca-Al mapping for casting process, before and after addition of Ca in ladle (Plant C, basic). MS&T Foundry In the heat carried out at the Missouri S&T foundry, medium-carbon (4340) steel was melted in a 100 lb IF. Aluminum was added in the furnace just before tap followed by a calcium silicon addition in the tap stream for deoxidation. Figure 16 show the area fraction covered by inclusions and the dissolved and total oxygen for samples collected during various stages of the casting process. Figures 16 show the ternary plots for sulfides and oxides, before and after Ca addition in the ladle.

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Figure 16: Comparison of (a) inclusion volume and (b) dissolved and total oxygen measured in samples collected at various stages of the casting process (MS&T foundry).



Alumina inclusions increased after the addition of Al in the furnace, with a significant increase in the total oxygen also. With Ca-treatment in the ladle, most of the alumina inclusions form calcium aluminates (CA), but the MnS inclusions remain almost constant and are not completely modified to CaS inclusions. This is due to the ineffective addition of CaSi during tap in small ladle, which does not allow the Ca to interact with the melt before vaporizing.14 These results are also consistent with the chemical mapping of the sulfides and oxides as shown in Figure 17. The oxide inclusions, and the total oxygen, increased from the ladle to the casting, indicating that there was significant reoxidation during the pouring.

Calcium


a)

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Figure 17: a) Sulfides Mn-Ca-S and b) oxides (Mn+Si)-Ca-Al mapping for casting process, before and after the addition of Ca in ladle (MS&T foundry).

Comparison Of The Plants Figure 18 compares the area fraction covered by inclusions, the average spacing between the inclusions and the total oxygen in the final cast product among all the plants, as measured by the MQA. Steel cleanliness is indirectly proportional to the area of inclusions in the sample and directly proportional to the inclusion spacing.4 The steel melted at the MS&T foundry had the highest area fraction of inclusions and the total oxygen in the cast product, followed by Plant A, Plant B, Plant C-acid and Plant C-basic. The direct correlation between the area fraction of inclusions and the total oxygen shows that the majority of the inclusions retained in the cast products were oxide inclusions. As the same area was analyzed for each specimen, the plant with the highest fraction of inclusions also has the lowest average spacing and indicates that the inclusions are densely populated. Since MS&T foundry had the smallest capacity ladle, there would not have been sufficient time for the inclusions to float out, which explains the highest number of inclusions in that sample. Plant A also had a relatively small ladle and there was no Ca-treatment, which caused high inclusions in the casting. Plant B had a larger ladle than Plant A and also used Ca-treatment in the ladle, thus, the casting had lesser inclusions than Plant A. Plant C had the highest capacity ladle and also used effective Ca wire treatment, so it had the minimum inclusions. Also, basic practice in Plant C produced lesser inclusions than the acid practice. Thus, Plant C with the basic EAF produced the cleanest steel among all the plants.

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Figure 18: Comparison of (a) inclusion volume, (b) total oxygen, and (c) average inclusion spacing in the samples from cast products collected from the different plants.

CONCLUSIONS

In this research, a new Metals Quality Analyzer is used for calculating the volume fraction and spacing of inclusions in steel casting. The MQA provides an automated, efficient and fast way to characterize and measure the non-metallic inclusions. The volume fraction and composition of the inclusions changes significantly during the melting and treatment of cast steels in foundries, which is primarily due to early deoxidation and further reoxidation during pouring. Steel cleanliness was found to be dependent on the size of the ladle and the deoxidants added. Ca deoxidation was found to be beneficial when added as wire and in sufficient quantities. It modified MnS to CaS with globular morphology, which is less harmful to physical properties. Ca also transformed alumina to calcium aluminates (CA) with lower melting point and spherical morphology, so they could easily float out, resulting in cleaner steel. The total oxygen in steel was found to be directly correlated to the volume of inclusions, showing that it is also an important index of evaluating steel cleanliness. Also, the steel with the lowest volume of inclusions had the highest average spacing between the inclusions and, hence, was the cleanest.



ACKNOWLEDGEMENTS The work for this project was made available through funding provided by U.S. Army Benet Labs Award W15QKN-07-2-0004 and the funding for the Aspex Corporation inclusion analyzer was made available through U.S. Army DURIP Grant W911NF-08-1-0267. The authors also acknowledge the support of the Steel Founders Society of America and the member companies that participated in this research.

REFERENCES

1. L. Zhang, B.G. Thomas, “Literature Review: Inclusions in Steel

Ingot Casting”, Metallurgical and Materials Transactions B, Vol.37B, No.5, 2006, pp.733-761.

2. T.B. Cox and J.R. Low, “Investigation of the Plastic Fracture of AISI 4340 and 18 Ni-200 Grade Maraging Steels”, Met Trans A, 5A, 1974, pp.1457-1470.

3. T.J. Baker and J.A. Charles, “Effect of Second-Phase Particles on the Mechanical Properties of Steel”, M.J. May, ed., The Iron and Steel Institute, Scarborough, United Kingdom, 1971, pp. 88-94.

4. W.M. Garrison, “Controlling Inclusion Distributions to Achieve High Toughness in steels”, AIST Trans, 4(5), 2007, pp.132-139.

5. S. Chakraborty and W. Hill: 77th Steelmaking Conf. Proc., ISS, Warrendale, PA, 1994, 389.

6. R.A. Rege, E.S. Szekeres and W. D. Forgeng: Met. Trans. AIME, 1, No. 9, 1970, pp.2652.

7. L. Zhang, B.G. Thomas, “Literature Review: State of the Art in Evaluation and Control of Steel Cleanliness”, ISIJ International, Vol. 43, No. 3, 2003, pp. 271-291.

8. Moore and Bodor, “Steel deoxidation practice: Special emphasis on heavy section steel castings”, AFS Transactions, 93, 1985, pp. 99-114.

9. Herrera et al, “Modification of Al2O3 inclusions in medium carbon aluminum steels by AlCaFe additions”, Ironmaking and Steelmaking, 33(1 51), 2006.

10. E.T. Turkdogan, Fundamentals of steelmaking, 1996. 11. B. Allyn, J. Carpenter, and B. Hanquist, “Deoxidation in Heavy

Section Steel Castings”, Harrison Steel Castings Company, SFSA Technical and Operating Conference, 2006.

12. R. Kiessling: Met. Sci., 15, No. 5, 1980, pp.161. 13. Aspex Corporation, 2003. 14. The Making, Shaping and Treating of Steel, Association of Iron and

Steel Engineers, 1985.

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The Metals Quality Analyzer MQA - Thermo Fisher …...The Metals Quality Analyzer MQA Process design of inclusion modification in cast steel using automated inclusion analysis Vintee