ELECTRO-SLAG REMELTING OF AISI M41 HIGH-
SPEED TOOL STEEL SCRAP

Taha Mattar

Steel and Ferroalloys Laboratory

Central Metallurgical Research and Development Institute, CMRDI,

P.O. 87 Helwan, Cairo

Egypt

Abstract

Tools of pipes’ internal threading manufactured from AISI M41 high-speed
tool steel.The scrap of these tools can not be reformed to a lower dimensions
tool due to their complicated form. On the other hand AISI M41 steel con-
tains expensive and sensitive alloying elements such as vanadium, chromium,
molybdenum and tungsten. The traditional melting process of such scraps in-
volves melting in induction furnace and remelting consumable electrodes by
electro-slag remelting process which results in high losses in the oxidizable
expensive alloying elements and high cost due to the melting and refining
processes. In this work one step recycling process by ESR was applied to
minimize production costs and time in addition to minimizing the losses in
the expensive alloying elements. The tool parts were welded in sequence and
used as consumable electrodes. Then was Electro-slag remelted using rec-
ommended calcium fluoride pre- fused slag to obtain clean and sound ingots
to be forged in the proper shape to form pipes’ threading tools. The produced
ingot by the one step recycling process has the advantage of minimum losses
in alloying elements by saving the losses in induction furnace melting and the
corresponding costs of alloying elements and melting process. The produced
ingot is sound, homogenous and has minimum amounts of sulphur compared
with the used scrap.

INTRODUCTION

AISI M41 steel grade is an item of the high speed tool steel group which

is one of the most important grades in tool steels. It is classified as super

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6TH INTERNATIONAL TOOLING CONFERENCE

hard fast cutting tool steel. It is characterized by its excellent mechanical
properties especially high hot hardness which leads to long durability of
tools. Due to its valuable technological properties it is used for heavy duty
applications as cutting, threading and turning tools [1, 2, 3].

Tool steels are usually manufactured by melting in electric arc furnace

(EAF) or induction furnace (IF) and the refining using secondary process
as electro-slag remelting (ESR), vacuum arc remelting (VAR) or vacuum
induction furnace (VIF). Ladle treatment process (LT) could be used after
melting the steel in EAF and in conjunction with it [1, 2, 3, 4, 5, 6, 7,
8, 9, 10]. These techniques are essential in tool steels manufacturing to
obtain the minimum content of sulphur, non-metallic inclusions and gases
in addition to the chemically homogenous steel tools which determine the
working efficiency of the tools [1, 2, 3, 8, 9, 10, 11, 12]. As the manufacturing
processes of these steels are complicated and sophisticated techniques, the
production cost of these steels from their scrap is a very expensive process.
The cost of manufacturing process includes the production cost in EAF or IF
and LF and the refining cost in ESR, VAR or VIF and the losses of alloying
elements as Vanadium, Chromium, Molybdenum, Silicon, and cobalt [3, 11].
The losses in alloying elements in each step should be substituted which
forms another cost item.

This work aims at recycling of AISI M41 high speed steel in one step

technique by ESR process instead of traditional 3 steps recycling technique
to reduce the production cost.

EXPERIMENTAL WORK

With the objective of this work AISI M41 high speed tool steel scrap

was recycled to be reused as expensive valuable material. This tools scrap
was collected and classified according to the shape and size, and then the
chemical investigation of each class was carried out.

The tool parts within the same class were welded together perpendicular to

the longitudinal axes using suitable welding electrodes to form consumable
electrode that will be remelted using ESR technique. The formed electrode
was used as consumable electrode in ESR unit with the electrical parameter
as shown in Table 1.

The ESR process is a special refining and remelting to make the sound

ingot of less impurity and non-metallic inclusions (NMI) with good quality,
by making the best use of the physical and chemical properties of slag.

Electro-Slag Remelting of AISI M41 High-Speed Tool Steel Scrap

405

Table 1.

Electrical data of the used remelting process

Main current

5.3 kA

Melting current

1.5 – 2.0 kA

Melting voltage

40 – 50 V

Melting power

60 – 100 kW

This process has gradually been proven to be excellent as an ingot making
technique of high grade steels.

The principle of ESR furnace used in the present investigation is that

power supply is connected to the consumable electrode on one hand and to
the mould electrode on the other hand. The consumable electrode is melted
by turning on electricity through the fused slag; 1.5 kg slag / 10 kg steel,
and the ingot is made in a water- cooling copper mould in a way of gradual
stacking solidification.

The used slag is pre-fused calcium fluoride based slag; CaF

2

/CaO/Al

2

O

3

(CCA) : 70/15/15; which is pre-fused in single electrode 5 kg arc furnace at
a rate of 450 A and 30 V, then crushed to 0.5 – 3 mm size. The melting tech-
nique used in this study is solid start technique where an exothermic bomb
composed of stochiometric mixture of iron oxide and aluminum powder was
used. This bomb is the item that close them for the electrical circuit.

The produced ingot was cleaned from the bottom. Then the hardness

was measured and the chemical composition was determined to evaluate the
produced steel material. Samples from the scrap as well as the ingot were
prepared for NMI examination.

The production cost and the cost of the additives in EAF or IF and LF

were calculated to determine the economic effect of investigated technology

RESULTS AND DISCUSSION

With the objective of the current work the scrap of AISI M41 steel tools

was collected prepared for remelting in one step technique instead of multi-
steps traditional techniques. The process that was used in the one step
technique is ESR. In ESR CaF

2

based slag was used while the solid start

was applied. The produced steel ingot was evaluated in terms of chemical

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6TH INTERNATIONAL TOOLING CONFERENCE

Table 2.

The chemical composition of scrap and produced ingot

Chemical composition, wt. %

C

Si

Mn

P

S

Cr

Mo

Co

V

W

Fe

Scrap

0

.

96

0

.

21

0

.

30

0

.

022 0

.

025 3

.

98

4

.

86

4

.

7

1

.

85

6

.

98

Bal

.

Ingot

1

.

0

0

.

36

0

.

42

0

.

022 0

.

008

3

.

5

4

.

5

0

.

35

1

.

7

6

.

4

Bal.

Change,% 4

.

2

71

.

5

40

.

0

0

−

68

−

12

−

7

.

5

−

92

.

6

−

8

−

8

.

3

composition. Table 2 shows the chemical composition in wt.% of used scrap,
the produced steel ingot and the change in composition.

From the data listed in Table 2 and the standards in the literature, it is clear

that the obtained composition after ESR process is within the compositional
range of AISI M2 high speed tool steel [1, 2, 3, 6, 9, 10, 11, 12]. The
difference between the two steel grades; M41 and M2; is the presence of
cobalt and slightly the other alloying elements. From these data it is clear
that the cobalt was totally oxidized in ESR process, which is the same case
in melting the steel in EAF or IF.

By evaluating the produced steel ingot, the hardness was measured and it

was ranged between 56 and 58 HRC, which means that used scrap, ESR
process (as one step manufacturing technique) and produced steel ingot
composition are suitable for tool manufacturing.

Concerning the effect of ESR process on the cleanliness of produced

steel ingot, from Table 2 it is clear that about 64% of the initial sulphur
content could be removed by refining the consumable electrode under the
CCA; 70/15/15 slag. That this kind of slag has a very high desulphurizing
power, this result agrees to large extent with the literature [3, 9, 10]. The
used slag not only has the high desulphurizing power but also it has a great
effect on the non metallic inclusions (NMI) content, size and distribution
in steel. The examined steel samples showed a great difference in the NMI
distribution and size before and after ESR process. The obtained data in
this study showed a better behaviour of NMI than previous studies that used
the EAF or IF for melting of scrap producing consumable electrodes. The
enhancement in NMI behaviour result from the fact that there are exogenous
NMI contaminate the molten steel during both EAF or IF and LF processes.

Electro-Slag Remelting of AISI M41 High-Speed Tool Steel Scrap

407

In case of one step ESR technique, the contamination of steel with the

additional amount of NMI was avoided. As the sulphur and NMI inhibit and
retard the efficiency of tools, so the removal of these impurities from steel
by ESR technique improves the technological properties of tool steels.

To determine the economic effect of one step technique it is important to

calculate and compare the difference in production cost in both one step and
multi steps techniques. The ESR step is common for both techniques, so the
difference in cost will arise from the EAF or IF melting of scrap in addition
to LF treatment. Table 3 summarizes the amount of alloy additives (either
in EAF or IF) and their cost concerning the production of 100 kg of steel
ingots.

In addition to the cost of added alloys for adjusting the composition, there

are the cost of consumables, power and operators cost, Table 4 summarizes
the total cost of melting 100 kg scrap in EAF or IF and casting in the form of
consumable electrodes in comparison with the preparation of scrap for one
step technique by welding to form consumable electrodes, where M: refers
to multi steps traditional melting technique; and O: refers to one step ESR
melting technique.

Table 3.

The cost of added alloys to 100 kg in EAF or IF process

Element

Used Alloy

Added wt., kg

Price, USD$

V

Fe-V

1.5

7.50

Cr

Fe-Cr

1.5

3.00

Si

Fe-Si

0.25

0.25

Mn

Fe-Mn

0.25

0.25

Mo

Fe-Mo

4.5

22.00

W

Fe-W

3

15.00

Total

48.00

As shown in Table 4, the usage of one step technique result in saving

183 USD$/ 100 kg melt and 1830 USD$/ ton melt which form a positive eco-
nomic impact for the suggested technique. On the other hand the usage of the
suggested technique produces a tool steel ingot with the production cost of
about 40 USD$/ 100 kg melt; i.e. 400 USD$/ ton melt; which is competitive
production cost of such steel grades on this scale.

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6TH INTERNATIONAL TOOLING CONFERENCE

Table 4.

The economic impact of one step technique for 100 kg

Item

Man Hour

Cost, USD$

Save in cost using one step

M

O

M

O

technique, USD$

Engineer

3

—

45

—

45

Technician

9

3

90

30

60

Consumables
and power

40

10

30

Additives to the
melt

48

—

48

Total

223

40

183

CONCLUSIONS

This work aimed at studying the possibility of recycling AISI M41 tool

steels scrap by one step technique through ESR process. The study showed
good results which can be concluded in:

Recycling of AISI M41 tool steels scrap by ESR succeeded producing
high value tool steel grade; AISI M2.

Usage the one step ESR technique results in a big reduction in the
amount of NMI and improves their distribution in comparison with
multi steps traditional techniques as a result of avoiding the contami-
nation by exogenous NMI.

A good economic impact arises from using the suggested technique
as the production cost could be reduced to be about 400 USD$/ ton
producing a save in production cost by amount of 1830 USD$/ ton.

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Electro-Slag Remelting of AISI M41 High-Speed Tool Steel Scrap

409

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