How ESR Improves Tool Steel Cleanliness for Mold and Die Applications

Electroslag remelting, usually called ESR, improves tool steel in two main ways:

• Molten slag removes or changes harmful non-metallic inclusions.
• The steel freezes again under controlled conditions, which can reduce severe segregation and produce a more consistent ingot.

These two effects solve different problems. Slag refining mainly deals with oxides, sulfides, and slag-related particles. Controlled solidification mainly affects how evenly alloying elements and the cast structure are distributed through the ingot.

For a mold or die, the results are practical. A large oxide can fall out during mirror polishing and leave a pinhole. A long sulfide stringer can weaken the steel across the forging direction. Segregation can make one area harder, softer, richer in carbides, or more difficult to polish than another.

ESR reduces these material-related risks. It cannot correct the wrong steel grade, poor heat treatment, sharp die corners, deep EDM damage, incorrect polishing, or poor operating conditions. It should be treated as one part of the complete mold steel selection and production process, not as a complete quality guarantee.

What Tool Steel Cleanliness Means

Clean steel does not mean inclusion-free steel. Commercial steel always contains some non-metallic particles.

Steel cleanliness describes:

• How many inclusions are present
• How large they are
• What they contain
• Whether they are round, sharp, cracked, or stretched
• Whether they are evenly spread or grouped in clusters
• Where they are located inside the steel

ASTM E45-25 contains recognized methods for describing the inclusion content of wrought steel. In its microscopic methods, inclusions are grouped mainly by their shape and appearance, not necessarily by their exact chemical identity.^[1]

ISO 4967:2026 provides another micrographic method. It applies to rolled or forged steel products with a reduction ratio of at least 3 and allows comparison with reference images or direct measurement by image analysis.^[2]

Cleanliness, Homogeneity, Soundness, and Carbides

These four quality terms are related, but they do not mean the same thing.

ESR directly affects inclusion cleanliness. When the molten pool and cooling process are properly controlled, it can also improve homogeneity and internal soundness.

Its effect on carbides is less direct. Carbide size and distribution also depend on the chemical composition, ingot size, cooling rate, heat treatment, and later forging.

A steel block can therefore have a good microscopic inclusion rating but still contain a large ultrasonic indication. Another block may pass ultrasonic inspection but still contain segregation that causes uneven heat-treatment or polishing results.

Indigenous and Exogenous Inclusions

Inclusions can be divided into two groups according to where they come from.

Indigenous inclusions form inside the steel during deoxidation, alloy reactions, cooling, or solidification. Examples include aluminum-rich oxides, manganese sulfides, magnesium-aluminum oxides, titanium nitrides, and calcium aluminates.

Exogenous inclusions enter the steel from outside. Possible sources include:

• Entrapped furnace, ladle, or casting slag
• Broken furnace or ladle refractory
• Oxide scale from an exposed electrode surface
• Dirty pouring equipment
• Large particles already present in the consumable electrode

Studies of non-metallic inclusions show that their origin, chemistry, size, and distribution depend on both the steel composition and the full melting and casting process.^[3]

Exogenous inclusions can be especially dangerous because they may be large, irregular, and randomly located. A small metallographic sample may miss them even when one exists elsewhere in a large block.

ASTM E2283 uses statistical methods to estimate the distribution of the largest indigenous inclusions. The standard states that it is not suitable for unpredictable exogenous inclusions and that broader inspection methods, such as ultrasonic testing, are needed to locate them.^[4]

Why Inclusion Size Matters

A large inclusion interrupts more of the continuous steel around it.

The inclusion and the surrounding steel do not deform in exactly the same way when the tool is loaded or heated. Stress builds around their boundary. If the inclusion is large, sharp, cracked, or poorly bonded, a small crack may begin there.

The largest inclusion can therefore be more important than the average inclusion size.

A large inclusion can cause different problems depending on its location:

• In a polished cavity, it can fall out and leave a pinhole.
• Near a cooling channel, it can weaken an already thin wall.
• At a sharp corner, it adds to the stress already created by the geometry.
• In a hot-work die, it can help a fatigue crack begin.
• In a cutting or forming die, it can increase the risk of chipping.

Larger particles may sometimes be easier to remove during ESR because they respond more strongly to liquid flow and buoyancy. This does not mean that all large particles are removed. Any large particle left in the steel can still be a serious risk.

Why Inclusion Shape and Chemistry Matter

A round particle normally produces less stress concentration than a sharp particle of the same size. Shape cannot be judged alone, however. A large, round, hard oxide may still damage a polished surface or start a fatigue crack.

Aluminum-rich oxides and magnesium-aluminum oxides are hard and brittle. During polishing, the steel around them may be removed faster than the particle. The particle can then stand above the surface, scratch nearby steel, break into smaller pieces, or fall out.

Manganese sulfides are softer. A controlled amount can improve machining because sulfides help chips break and reduce friction near the cutting edge.

During forging or rolling, sulfides can stretch into long stringers. Research has linked sulfide inclusions with changes in fatigue strength, fracture toughness, notch toughness, transverse tensile properties, and differences between the rolling and transverse directions.^[5]

Long sulfide stringers may therefore:

• Reduce properties across the forging direction
• Leave streaks during polishing or chemical texturing
• Make mechanical properties more directional
• Support local corrosion in some stainless mold steels

Inclusions and Carbides Are Different

Non-metallic inclusions are generally unwanted particles formed during steelmaking or introduced from outside the steel.

Carbides are hard phases formed when carbon combines with chromium, molybdenum, vanadium, tungsten, or other alloying elements. Many tool steels are designed to contain carbides because they provide wear resistance and help the steel keep its hardness.

ESR does not remove carbides in the same way that molten slag can absorb an oxide inclusion. ESR mainly changes the solidification conditions that control where alloying elements and carbides collect.

This distinction is important in high-carbon cold-work grades such as D2 and 1.2379 cold-work tool steel. In these grades, coarse carbide groups and carbide bands may affect wear and chipping more than ordinary microscopic oxide inclusions.

Powder-metallurgy production is often selected when fine and evenly distributed carbides are the main goal. ESR is often selected when inclusion cleanliness, large-section consistency, toughness, and polishability are more important.^[6]

How ESR Removes Inclusions

ESR starts with a consumable steel electrode that already contains the required basic alloy composition.

The electrode is lowered into molten slag inside a water-cooled copper mold. Electric current passes through the slag. Because the slag has high electrical resistance, it becomes hot enough to melt the bottom of the electrode.

A thin film of liquid steel forms on the electrode tip. This liquid gathers into droplets, passes through the slag, and enters a molten-steel pool. The steel then freezes inside the water-cooled mold.

ESR slag does not work like a physical screen. It does not filter molten steel through small holes.

An inclusion is removed only when:

• It moves to the boundary between the steel and slag.
• The slag can accept, dissolve, or react with it.
• The steel remains in contact with the slag long enough.
• The inclusion stays in the slag instead of returning to the steel.

The Liquid Film on the Electrode

The first refining area is the thin film of molten steel on the electrode tip.

Published ESR research has reported film thicknesses of about 50–200 μm under the conditions studied. Because the film is thin, an inclusion has only a short distance to travel before reaching the surrounding slag.

Slag movement also washes the surface of the liquid film. This renews the contact area between steel and slag.

Refining at this stage works best when:

• The film remains in contact with the slag for enough time.
• The inclusion can move to the film surface.
• The slag can dissolve or hold the inclusion.
• The temperature and slag flow remain stable.

If the electrode melts too quickly, the liquid film may have less time to react with the slag.

Droplets Passing Through the Slag

After leaving the electrode tip, steel droplets fall through the slag.

Reported ESR droplet diameters are commonly about 1–10 mm, although the real size depends on the electrode diameter, current, slag properties, and operating conditions.

Smaller droplets have more surface area compared with their volume. This can improve contact with the slag. However, the smallest possible droplets are not always the best target.

Droplet size also affects:

• How quickly the droplet falls
• How long it stays in the slag
• How liquid moves inside the droplet
• How heat moves between the steel and slag
• How stable the electrical process remains

The practical target is a stable and repeatable droplet pattern, not simply the smallest possible droplet.

What One H13 Study Found

One published study examined H13 die steel remelted in a 300 mm ESR mold. The slag contained 65% CaF₂, 30% CaO, and 5% Al₂O₃. The process was carried out without a protective atmosphere.

The researchers used electrolytic extraction to measure inclusions larger than 50 μm.

When the slag mass increased from 17.85 kg to 20.50 kg, the measured mass of these large inclusions decreased by an average of 66.18%. The two separate test groups showed reductions of 75.77% and 56.59%.

The researchers connected the result with less electrode oxidation, a longer path through the slag, and stronger slag movement.^[7]

This result does not mean that adding the same amount of slag will reduce inclusions by 66.18% in every ESR furnace. It applies only to the steel, slag, mold, equipment, and test conditions used in that study.

The Molten-Steel Pool

After passing through the slag, the droplets enter a molten-steel pool.

An inclusion that moves upward may return to the slag. A particle carried deeper into the pool may become trapped in the ingot as the steel freezes.

Its path depends on:

• Particle size and density
• Pool depth
• Liquid-steel movement
• Electromagnetic forces
• The position of the freezing boundary
• How quickly the steel solidifies

A shallow and stable pool usually gives inclusions a shorter route back to the slag. A deep pool gives inclusions and alloy-rich liquid more space to move toward the center.

Why Slag Chemistry Matters

ESR slag is a designed refining material, not waste slag from another furnace.

Common ESR slags contain controlled amounts of calcium fluoride, calcium oxide, aluminum oxide, magnesium oxide, and other components.

The slag must perform several jobs:

• Generate heat through electrical resistance
• Remain liquid at the working temperature
• Flow well enough to contact the steel
• Absorb or dissolve inclusions
• Remove sulfur when required
• Protect the steel from air
• Control heat transfer
• Lubricate the space between the ingot and copper mold

Research on ESR slags shows that their chemical composition strongly affects melting temperature, viscosity, electrical behavior, inclusion absorption, and the final quality of the ingot.^[8]

There is no single slag formula that is best for every steel. A slag used for H13 or 1.2344-type hot-work steel may need different control from one used for a high-chromium stainless mold steel.

Slag Viscosity

Viscosity means how easily the slag flows.

If the slag is too thick:

• Droplets and inclusions move more slowly.
• Steel-slag contact is less effective.
• The liquid film on the electrode is washed less strongly.

If the slag is too thin, control of the slag skin, heat transfer, and electrical process may become more difficult.

Viscosity also changes with temperature. A slag that flows correctly at its normal working temperature may become too thick if the temperature falls.

Slag Basicity and Oxygen Control

A more basic slag can improve sulfur transfer and help absorb some oxide components. However, adding more calcium oxide does not automatically improve the steel.

It can also change:

• Slag viscosity
• Electrical resistance
• Melting temperature
• Reactions with aluminum, silicon, titanium, and other alloying elements

If the slag has too much oxidizing power, reactive alloying elements can oxidize and form new inclusions.

Inclusion Modification

Some inclusions are removed completely. Others change composition during ESR.

For example, an aluminum-rich oxide may react with magnesium, calcium, or other slag components. The new particle may have a different shape, hardness, or melting temperature.

Useful modification may make an inclusion:

• More rounded
• Easier to dissolve in slag
• Less likely to stretch during forging
• Less damaging to the steel

Modified does not mean harmless. A hard magnesium-aluminum oxide or complex slag particle can still cause polishing or fatigue problems.

Protected-Atmosphere ESR

Protected-atmosphere ESR uses a controlled gas around the remelting area.

Its main purposes are to reduce:

• Reoxidation
• Nitrogen pickup
• Oxidation of the hot electrode
• Loss of reactive alloying elements

Hydrogen control also requires dry slag, dry equipment, a clean electrode, and low-moisture gas. A protective atmosphere cannot correct wet slag or poor furnace preparation.

Desulfurization

Sulfur can move from molten steel into a suitable slag.

This transfer may take place:

• In the liquid film on the electrode
• Around falling droplets
• At the surface of the molten pool
• Through reactions between the slag and furnace gas

ESR desulfurization depends on the starting sulfur content, slag chemistry, atmosphere, deoxidation method, melting rate, and electrical settings. It does not remove the same percentage of sulfur in every process.^[9]

This trade-off can be seen by comparing 1.2083 stainless mold steel, which is used where corrosion resistance and polishing are important, with 1.2085 sulfur-alloyed stainless mold steel, which gives more priority to machining efficiency.

How ESR Controls Solidification

Removing inclusions is only part of the process. The refined steel must also freeze into a sound and reasonably uniform ingot.

Why Melting Rate Matters

A higher melting rate produces more steel per hour, but it also sends more heat into the molten pool.

In one numerical study of high-carbon martensitic stainless steel, increasing the melting rate from 90 kg/h to 180 kg/h produced the following calculated changes:

These results apply to the modeled steel, ingot, and furnace conditions. They are not universal ESR operating limits.^[10]

The useful lesson is simple: melting too quickly can deepen the pool, slow cooling at the center, produce a coarser cast structure, and increase segregation.

How Segregation Forms

As molten steel freezes, all alloying elements do not enter the solid steel at the same rate.

Some carbon, chromium, molybdenum, vanadium, and other elements remain in the liquid between growing crystals. The last liquid to freeze can therefore have a different composition from the steel that froze first.

A dendrite is the tree-like crystal shape that forms as steel freezes. Alloy-rich liquid can remain between its branches. Carbides may later form in these areas.

Segregation can change:

• Carbide amount and size
• Hardness after heat treatment
• Hardenability
• Tempering response
• Toughness
• Polishing and etching behavior
• Corrosion response

Macrosegregation means a large difference, such as a difference between the center and surface of an ingot.

Microsegregation means a small local difference between nearby dendrites.

Well-controlled ESR can reduce severe macrosegregation compared with ordinary ingot production. It cannot remove all segregation. If the molten pool is too deep or unstable, center segregation can still occur.

Why Ingot Size Matters

The same ESR label does not mean that a small bar and a very large mold block have the same internal quality.

As the ingot becomes larger:

• The center is farther from the water-cooled mold.
• Heat takes longer to leave the center.
• The molten pool is harder to keep shallow.
• The center and surface freeze under more different conditions.
• Melting rate and cooling become harder to control.

Data from a small laboratory bar should not be used as proof of the center quality of a large industrial mold block.

For a large block, the buyer should request test results from a similar ingot size, forged section, sampling position, and test direction.

Why Forging After ESR Still Matters

An ESR ingot has a cleaner and more controlled cast structure, but it is still a cast ingot.

Forging or rolling is used to:

• Break down coarse cast grains
• Break up or spread carbide groups
• Close some internal pores
• Improve structural continuity
• Reduce retained cast structure at the center
• Convert the ingot into a plate, bar, or mold block

A high stated forging ratio does not automatically prove good center quality. The result also depends on heating temperature, press capacity, deformation at the center, and final forging temperature.

ESR improves the starting ingot. Forging determines how much of that improvement remains in the finished block.

How Cleanliness Affects Molds and Dies

Mirror Polishing

Mirror polishing reveals defects that normal machining can hide.

During final polishing, only a thin layer of steel is removed. Small differences in inclusion hardness, carbide content, and local steel hardness can become visible.

Inclusion Pull-Out

An inclusion may not be strongly bonded to the surrounding steel. Polishing removes material around its edge until the particle becomes loose and falls out.

The empty space appears as a pinhole. More polishing may make the hole wider because additional steel is removed around it.

Hard Particles

A hard oxide or carbide may resist polishing while the softer steel around it is removed.

The particle can:

• Stand above the surface
• Scratch nearby steel
• Break into smaller abrasive particles
• Fall out and leave a pit
• Create a bright or dark point under reflected light

Research on polished tool-steel surfaces has shown that hard inclusions can form small raised areas, while softer inclusions such as MnS can form shallow craters because they are removed at different rates from the steel around them.^[11]

Segregation Bands

If one area contains more carbides or has a different hardness, it may polish faster or slower than the surrounding steel.

This may appear as:

• Cloudiness
• Streaks
• Waviness
• Uneven reflection
• Visible structural bands

High-cleanliness stainless grades such as S136 are commonly considered for transparent and high-gloss parts. The article on S136 steel for high-gloss molds explains where this type of steel is normally used.

Not Every Polishing Defect Is Caused by Steel Cleanliness

ESR can reduce defects caused by the material, but it cannot replace correct grinding and polishing. The article on surface grinding in mold steel processing explains the machining controls needed before final polishing.

Chemical and Laser Texturing

Chemical texturing removes different areas of the steel at different rates.

A segregated or carbide-rich band may etch deeper or shallower than the surrounding steel. Inclusions may also dissolve or fall out, leaving small pits.

On a large textured mold, this may cause:

• Visible lines
• Uneven texture depth
• Patchy gloss
• Different appearance between inserts
• The same visual defect on every molded part

Laser texturing depends less on chemical reaction, but local hardness, carbide content, and heat response can still affect the result.

When several inserts form one visible surface, they should preferably have the same heat number, material direction, heat-treatment condition, and surface preparation.

Weld-repaired areas should be tested before the complete cavity is textured because the weld metal may react differently from the original ESR steel.

Hot-Work Dies

Hot-work dies repeatedly experience heating, cooling, pressure, erosion, and mechanical loading.

Cleanliness affects surface heat checking and large cracking in different ways.

Heat Checking

Heat checking is a network of small surface cracks caused by repeated heating and cooling.

The surface becomes hot and tries to expand while the colder steel below holds it back. During cooling, the surface contracts. Repeating this cycle eventually produces cracks.

The main factors usually include:

• Surface temperature
• Temperature change during each cycle
• Die preheating
• Cooling-channel design
• Spray and lubricant practice
• Cycle time
• Steel hardness
• Tempering resistance
• Residual stress
• EDM, nitriding, and surface condition

An inclusion near the surface can help a crack begin, but cleanliness is normally only one supporting factor in heat checking.

Gross Cracking

Gross cracking is a larger crack that can stop production or break a major section of the die.

Cleanliness has a more direct connection with this type of failure because a large inclusion or cluster can create a serious local weak point.

The risk is higher when the inclusion is close to:

• A sharp corner
• A thin cavity wall
• A cooling channel
• A screw hole
• A weld repair
• An area under high tensile stress

Long inclusions and structural bands are especially important when stress acts across the main forging direction.

Transverse testing is performed across the forging direction. Short-transverse testing is performed through the block thickness and often gives a lower toughness value than longitudinal testing.

For a critical die-casting block, do not accept only a longitudinal impact value. Ask for the test direction, sampling position, original block size, heat-treatment condition, and actual heat-specific results.

Corrosion-Resistant Molds

ESR does not create corrosion resistance by itself. Corrosion resistance mainly comes from the steel composition, especially chromium, together with correct heat treatment and surface condition.

Cleanliness can make corrosion performance more consistent.

Research on stainless steel shows that MnS and oxy-sulfide inclusions can start localized corrosion when the area around the inclusion becomes chemically aggressive.^[12]

A hole left by a dissolved or detached inclusion can hold moisture, chlorides, cooling water, or chemical residue from the plastic.

Severe segregation can also create local differences in chromium and carbide condition.

For PVC, flame-retardant plastics, or other chemically aggressive materials, a high-chromium grade such as 1.2083 stainless mold steel is normally more suitable than relying on ESR treatment of a low-chromium steel.

Final corrosion performance still depends on:

• Steel grade and chromium content
• Hardening and tempering
• Carbide condition
• Surface finish
• Cooling-water quality
• Plastic additives and released gases
• Cleaning and storage conditions

How to Verify ESR Steel Quality

The letters “ESR” describe a production process. They do not prove a specific inclusion rating, ultrasonic quality level, toughness value, or center quality.

No single inspection method can find and identify every type of defect.

SEM-EDS

A scanning electron microscope, or SEM, shows the shape and fine structure of an inclusion.

Energy-dispersive X-ray spectroscopy, or EDS, identifies the main elements in the particle.

ASTM E2142-08(2023) describes SEM-based methods for measuring inclusion size distributions and classifying inclusions by their chemical composition.^[13]

SEM-EDS can support likely classifications such as:

• Alumina-rich oxide
• Magnesium-aluminum oxide
• Calcium aluminate
• Manganese sulfide
• Titanium-rich nitride
• Slag-related particle
• Refractory contamination

EDS alone does not always prove the exact crystal phase. More detailed phase analysis may require other methods.

Ultrasonic Testing

Ultrasonic testing detects sound reflections from internal discontinuities.

These indications may be connected with:

• Large inclusions or inclusion clusters
• Porosity
• Shrinkage
• Laminations
• Cracks
• Other internal discontinuities

ASTM A388/A388M-26 covers contact pulse-echo ultrasonic examination of steel forgings using straight-beam and angle-beam techniques. It also requires the ultrasonic quality level to be clearly stated in the order or specification.^[14]

Ultrasonic testing normally cannot prove the exact type or chemistry of an indication. The result must be judged using the agreed standard, calibration, sensitivity, indication size, position, and product form.

Macroetch Examination

Macroetching shows large-scale structural conditions rather than individual microscopic particles.

ASTM E381-22 states that macroetching can reveal differences in grain or dendritic structure, segregation, banding, cracks, porosity, pipe, and other discontinuities. Its stated scope is limited to specified carbon- and low-alloy-steel product forms, so the inspection method must be checked against the actual tool steel and product.^[15]

What a Useful Inspection Report Should Include

A microscopic cleanliness report should state:

• The standard and edition
• The exact test method
• The sample position
• The sample direction
• The inspected area or number of fields
• The inclusion categories
• The actual result
• The agreed acceptance limit
• The heat number

An ultrasonic report should state:

• The inspection standard
• The required quality level or acceptance class
• The probe and reference sensitivity
• The scanned dimensions and coverage
• The location and size of reportable indications
• The heat or block identification

A statement such as “UT passed” is not enough without the inspection method and acceptance level. The guide on reading a mold steel mill test certificate explains how chemistry, hardness, ultrasonic results, and heat-number traceability should be checked together.

Typical values in a supplier brochure are not the same as heat-specific test results. Typical values describe normal product performance. Heat-specific results describe the actual batch supplied.

What to Specify When Buying ESR Tool Steel

Supplier capability should also be checked before ordering a high-value block. A mold steel supplier audit should verify material identity, storage, inspection equipment, machining capability, report traceability, and the handling of rejected material.

When ESR Is Worth the Extra Cost

ESR steel costs more because it requires another melting operation, controlled slag, extra energy, process monitoring, lower production yield, and additional inspection.

The correct comparison is not only the steel price per kilogram.

ESR is easier to justify when:

• A material defect would cause expensive repair or replacement.
• The steel cost is a small part of the complete mold cost.
• The mold requires mirror polishing or uniform texturing.
• The block is very large.
• The die works under severe mechanical or thermal loading.
• Properties across the forging direction are important.
• Production downtime is expensive.
• The replacement lead time is long.

Typical applications include:

• Optical and transparent plastic molds
• Mirror-polished cavities
• Large textured automotive molds
• Large die-casting dies
• High-cycle forging dies
• Thick corrosion-resistant mold blocks
• Tools that are difficult to repair or replace

ESR may provide limited value when:

• The tool is small and lightly loaded.
• The surface finish is not important.
• The production run is short.
• Abrasive wear is the main failure mode.
• A proven conventional grade already gives acceptable life.
• Tool design and heat treatment are not yet under control.

Use Total Tool Cost, Not Steel Price Alone

Compare the ESR premium with the cost of one possible material-related problem:

• Additional polishing
• Welding and re-machining
• Repeated texturing or coating
• Rejected trial parts
• Production downtime
• Delayed delivery
• A replacement block and new machining

If one repair event costs more than the ESR premium, reducing the chance of that event may justify the more expensive steel.

This does not mean ESR is always the cheapest choice. It means that the decision should be based on the total tool cost and the main failure risk, not only the raw steel price.

Conclusion

ESR improves tool steel through slag refining and controlled re-solidification.

The slag can remove or change oxide and sulfide inclusions. Controlled solidification can reduce severe segregation and create a better starting ingot for forging and heat treatment.

The main value of ESR is not a guaranteed large increase in one property. Its main value is reducing the chance of a serious local defect or an unexpected difference inside the steel block.

The final result still depends on the electrode, slag, atmosphere, melting rate, pool depth, ingot size, forging, heat treatment, and inspection.

For a low-risk tool, conventional steel may be sufficient. For a large, highly polished, heavily loaded, or expensive mold or die, verified ESR steel can reduce the risk that one hidden inclusion or one segregated area becomes the point where the tool fails.