GB2222178A - Vacuum arc re-melted steels - Google Patents

Abstract

A method of producing an alloy steel component substantially tree of manganese frill entrapment. The method comprises the step of subjecting an electrode produced from an alloy steel having a maximum manganese content of less than 0.10% by weight to vacuum arc re-melting.

Description

Vacuum Arc Remelted Steels This invention relates to vacuum arc remelted steels.

Vacuum arc remelting is a process used in the production of special quality alloy steels in which the steel is cast or forged into an electrode for re-melting in a vacuum by an electric arc struck between the electrode and a pool of molten droplets which collect on the surface of an ingot produced progressively by cooling of the molten steel present in the pool. Close control of the impurity content of the re-melted product can be achieved by the process.

The quest for higher integrity, structurally sound and defect "free" material has resulted in vacuum arc re melting being adopted for certain components used in, for example, the aerospace, nuclear and ordnance industries.

The change from air melting to vacuum arc re-melting for such components has resulted in a significant reduction in rejection rates whilst enabling components to be to designed with more confidence.

In general, the compositions used for vacuum arc remelted components are identical to those of their airmelt predecessors, other than slight changes made to compensate for losses in volatile elements.

Whilst significant improvements are achieved through the use of vacuum arc remelting, there still remains a finite rejection rate from, for example, scum entrapment, high manganese frill entrapment, and foreign metal contamination. This rejection rate has led to the quest for even better melting and re-melting procedures which has in turn led to the use of the vacuum induction melting process for producing the steel electrodes to be subsequently vacuum arc re-melted; this is the case particularly for the critical aerospace components (ie those components whose failure could result in the loss of an aircraft). Vacuum induction melting is a process in which liquid steel is treated within an evacuated highfrequency induction vessel to achieve a steel product low in impurity.The use of vacuum induction melting in addition to vacuum arc re-melting has resulted in a further reduction in the overall rejection rates.

No significant reduction in the rejection rate for high manganese segregates was however noted as a result of this latter change; indeed, in cases where re-melting rates were reduced to improve solidification structures a higher incidence of such segregates was recorded.

Examination of the origin and formation of these high manganese segregates and examination of the melting route and deoxidation procedures has indicated that it is possible to avoid such segregates, whilst maintaining the integrity of materials melted.

The present invention sets out to reduce significantly the presence of high manganese segregates by defining a family of vacuum arc re-melted steels in which the manganese content is reduced by a surprisingly substantial amount below the current minimum levels without any deterioration of the chemical and physical properties of the final product. According to the present invention in one aspect, there is provided a vacuum arc remelting alloy steel whose composition includes a maximum manganese content of 0.10% by weight.

According to the present invention in another aspect, there is provided an vacuum arc remelted alloy steel whose composition includes the following elements by weight:- up to 12% chromium; and upto 0.10% manganese.

According to the present invention in a still further aspect, there is provided an alloy steel component produced fro R steel as identified in either one of the preceding two paragraphs.

The invention will now to be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a partial section taker through conventional vacuum arc remeleting plant; and Figure 2 is an enlarged view of the encircled pert cf Figure 1.

Figure 3 shows the relationship between manganese content and manganese segregation.

As mentioned previously and as schematically illustrated in Figure 1, durina vacuum arc re-melting, metal passes from a consumable electrode 1 to a molten pool 2 in the form of molten droplets, these droplets being formed in the high temperatures present in and around an electric arc 3. Molten metal present in the pool solidifies progressively on cooling to form an ingot 4.

The process operates within the confines of a copper crucible 5, the configuration of which determines the cross-section of the ingot.

The reaction in the vicinity of the arc 3 is a violent one due to the very nature of the effect of currents in the region of 10-15 KA being passed across a small arc gap (typically 15mm) between the tip of the electrode 1 and the surface of the pool 2. Further disturbances of the morn ten pocl are brought abcut b cas evolved from the droplets and the pool and with conventional re-melted steels, evaporation of certain volatile elements such as manganese. A consequence of the violent reaction is that metal droplets are spattered against the cooled wall of the copper crucible 5. These droplets solidify to form a frill 6 which ch effectively becomes an extension of the crucible wall.The chilled frill 6 becomes an ideal receptacle for the condensation of volatile elements and, in particular, manganese.

Further spatter then occurs which results in a build up of droplets of refined retal coated with the manganese ric condensate.

Under normal, stable re-melting conditions, the manganese coated frill 6 becomes attached to the outerskin of the re-melted ingot to give a burnt appearance to the vacuum rerelted ingot. Subsesquent oxidation of the ingot surface during heating for forging, and machining after fcrging, completely removes this frill and thus eliminates any probability of material containing the manaanese contaminated frill entering service.

Unfortunately however, uniform re-melting conditions are not always achieved in practice and during periods of instability the depth of the molten pool can be observed to increase and decrease. This increasing and decreasing of the pool depth, coupled with vibrations resulting ro the violent reaction at and around the arc, can sometimes cause pieces of frill to be underlined by the molten pool, become detached fro the mould wall and fall into the solidifying pool.

Incomplete dissolution of these detached pieces results in areas of high manganese (known as "high manganese segregates") being present in the solidified Ingot. These segregates are undetectable by normal nondestructive testing methods such as Ultrasonic, l-lPI and DPI. They are only revealed by acid inspection of the surface of components, in which they appear as either alloy segregate regions or areas of variable grain size.

It is difficult to generalise on the effects of such segregates but they have been noted significantly to alter transformation products and when observed in components, their presence results in rejections.

As shown In Figure 3, the tendancy of steels to show hich manganese segregates is related to the manganese content cf the steel.

Hitherto, steels subjected to vacuum arc remelting typically have included manganese contents of th order of 0.5 by weight, some grades of steel including up to 1.7% by weight. The Applicants have now appreciated that the addition of manganese for de-oxidation purposes is not required in vacuum induction melted or vacuum treated steels because de-oidation is achieved by promotion of the carbcn-oxyaen reaction under the high vacuum employed.

In the absence of a relatively high manganese content, modification of transformation can readily be achieved by other non-reactive alloying additicns.

In the present invention, therefore, alloy steels to be subjected to vacuum arc re-melting have a relatively lo manganese content. Typically, this is of the order of 0.018 by weight. The maximun permissable manganese content is 0.10 by weight.

Exâ7F i es of typical alloy steels are as follows:- Lo alloy steels e.g. Carbon Chromium Steels (e.g. BS 970 534H99 grade!; lXi- Cr-Mo Steel (e.g. BS S82 types); Cr-Mo-V Steels, (e.g. BS S106, S134 types); 13% Cr steels (e.g. FV448, FV53 types) creep resistant steels (e.g.

M152, M153 types!.

Thus it is possible in vacuum arc re-melted materials to eliminate (or at least substantially reduce! the formation of high manganese segregates sirpiy, and surprisingly, by reducing to a minimun level the manganese content of the steel to be remelted. Such low manganese levels are not acceptable in air-melted materials of similar composition. Maintenance of the properties obtained in the conventional materials can cnly be achieved by slight modification of analysis balance.

It is believed that the introduction of alloy steels in accordance with the invention will have a significant effect in reducing rejection rates and increasing the safety of critical aircraft components, for eapl, discs in 12% chromium. steels, shafts in Cr Mo steels, gears and bearIngs in case hardening steels and through hardened tool steels.

It is to be understood that the foregoing is merely exemplary of certain embodiments of the invention and that modifications can be made thereto without departing from the true scope of the invention.

Claims (6)

CLAIMS:

1. A method of producing an alloy steel component substantially free of manganese frill entrapment, the method comprising the step of subjecting an electrode produced from an alloy steel having a maximum manganese content less than 0.10% by weight to vacuum arc remelting.

2. A method as claimed in Claim 1 wherein the alloy steel electrode has a manganese content of the order of 0.01% by weight.

3. An alloy steel component produced by a method as claimed in Claim 1 or Claim 2 from a vacuum arc re-melting alloy steel whose composition includes a maximum manganese content by weight of 0.10%.

4. A vacuum arc re-melting alloy steel as claimed in Claim 3 wherein the manganese content of the steel is of the order of 0.01% by weight.

5. A method of producing an alloy steel component having a maximum manganese content less than 0.10% by weight substantially as herein described.

6. A vacuum arc re-melting alloy steel having a maximum manganese content less than 0.10% by weight substantially as herein described.