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Hydrogen control of large bottom-poured forging ingots at Ellwood Quality Steels

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Hydrogen removal and hydrogen pickup during liquid steel processing at Ellwood Quality Steels are studied in detail with particular emphasis on the effect of LF-EMS electromagnetic stirring on hydrogen removal. The results of this work indicate that the ultralow-hydrogen practice implemented at Ellwood Quality Steels allows for consistent production of high-quality, large cross-section, bottom-poured steel ingots with hydrogen content of less than 1.5 ppm.

The Ellwood Group operates two separate electric arc furnace (EAF) meltshops, each with a maximum heat size of 47 metric tons: 

  • Ellwood Quality Steels (EQS) in New Castle, Pa., USA, with a capacity of 390,000 metric tons of plain carbon steel, low- and medium-alloy steel, tool steels and martensitic stain- less. Maximum ingot weight at EQS is 170 metric tons. 

  • Ellwood National Steels (ENS) in Irvine, Pa., USA, with a capacity of 80,000 metric tons of high-alloyed, low-car- bon stainless, Ni-based and other sophisticated alloys.1 Maximum ingot weight at ENS is 90 metric tons. 


The EQS meltshop was commissioned in late 1985 with a maxi- mum heat size of 40 metric tons.2 The heat size increased gradually to 47 metric tons with the following modifications: 

  • Original 1985 runner tap EAF replaced with enlarged eccentric bottom tapping (EBT). 

  • Lengthening of the original ASEA-SKF ladles. 

  • Optimization of ladle refractory thickness. 


With these heat size improvements, the maximum as-cast bottom-poured forging ingot weight increased to 47 metric tons from a single heat. Production flow at the EQS meltshop is shown in Fig. 1. 

The EQS meltshop has produced roughly 8,300,000 metric tons of forging and ring rolling ingots since start-up in December 1985. There are two teeming bays at EQS: 

  • West teeming bay for 24- to 47-metric-ton ingots teemed by overhead crane. 

  • East teeming bay for 2- to 24- and 60- to 170-metric-ton ingots. 


The EAF, ladle furnace No. 1 (LF1), vacuum station, ladle furnace No. 2 (LF2) and chemical laboratory (located between EAF and LF1) are very compact with a total steelmaking platform length of only 85 m. Both ladle furnaces are equipped with electromagnetic stirring (EMS) in order to allow for short-arc reheating under a fully liquid reducing slag cover. This makes it possible for EQS to consistently produce extremely clean steel with very tight composition control while maintaining high productivity.3 Table 1 lists some key performance indicators (KPIs) for 2017. 

Table 1: Key Performance Indicators, 2017

 KPI  Value
 EAF gross tap-to-tap time  52.7 minutes/heat
 EAF power-on time  36.1 minutes/heat
 EAF power-off time  16.6 minutes/heat
 Productivity  27.3 heat/day

The sandwich pouring process was implemented at EQS in 2015 in order to produce up to 170-metric ton ingots using four ladles of liquid steel. Sandwich pouring is shown schematically in Fig. 2. The development work and excellent quality results of sandwich-poured, large cross-section ingots at EQS are well documented.4

Fig 1: Production flow at Ellwood Quality Steels (EQS) meltshop

Fig 2: Schematic of sandwich pouring process.

Hydrogen in Steel

It was recognized in the early 20th century that certain internal hairline cracks in large steel forgings were related to hydrogen.5 These cracks have been termed “hydrogen flakes” and extensive research on their formation and prevention has been performed by both academia and industry.

Hydrogen is present in steel as a monatomic species with high diffusivity and low solubility in low- temperature-transformation products. The mechanism for hydrogen flake formation remains controversial, however calculations have been performed6 to show that the pressure buildup due to hydrogen with- in a steel matrix is easily high enough to exceed that which even a high-strength steel is able to withstand.

The atomic fraction of hydrogen in equilibrium with H2 gas at pressure P (atm) is given as: 7
 (Eq. 1)

where C0 is the atom fraction of hydrogen and T is in Kelvin. P must be replaced by fugacity at the pressures being considered. An approximation of the Taylor expansion can be used to estimate fugacity, f:
 (Eq. 2)

Where   is the molar volume and R is the gas constant. In order to determine the molar volume, the van der Waals equation of state for one mole of gas can be used:

    

(Eq. 3)

(Eq. 4)

(Eq. 5)

After determining the fugacity, the pressure and molar volume can be simultaneously solved. Fig. 3 shows the internal pressure buildup versus various amounts of hydrogen in the steel matrix at different temperatures. Fig. 4 shows the difference in relative volumes of steel, hydrogen gas, and water at standard temperature and pressure.


Fig 3: Internal pressure buildup from hydrogen

Fig 4: Steel, hydrogen and water volume

The important point from Figs. 3 and 4 is that even at a hydrogen content of 1 ppm, coming from a very small relative volume of water, the matrix will be unable to withstand the high internal pressure build- up at room temperature. The hydrogen present with- in the steel must be accommodated in some fashion. Hydrogen accumulates at voids and interfaces within the steel, thereby lowering the hydrogen dissolved within the matrix. Grain boundaries, dislocations, microporosity and inclusions are all potential trap- ping sites where hydrogen is able to diffuse out of the matrix and remain in these traps without detrimental flakes occurring.7 Fully dense forgings with low inclusion content are more susceptible to hydrogen flaking due to the reduced availability of trapping sites.

Hydrogen can be removed from steel forgings by subcritical diffusion annealing in order to prevent hydrogen flaking. However, the diffusion annealing practice is both time-consuming and expensive. Fig. 5 shows the required diffusion annealing time versus the forging diameter for removal of 50% of the original hydrogen content at 650°C according to Thelning’s calculation.8 The diffusion annealing time required for hydrogen removal in large cross-section forgings is prohibitively long.

Fig 5: Diffusion time for 50% hydrogen removal.

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