GB2133047A - Additive for aqueous quenching by immersion of aluminium-base alloys - Google Patents

Abstract

An agent for aqueous quenching of components of aluminium alloys, that have been previously raised to a temperature of at least 430 DEG C comprises, per litre of water, from 5 to 35g and preferably from 6 to 20g of polyvinyl pyrrolidone and from 50 to 250 g of an additive that causes reversible precipitation of polyvinyl pyrrolidone at the surface of the components at the moment when they are introduced into the quenching agent. The most effective additive is sodium chloride, in a concentration of from 100 to 250 g/l.

Description

SPECIFICATION Additive for aqueous quenching by immersion of aluminium-base alloys The present invention concerns an additive for aqueous quenching agents for aluminium-base alloys.

In order to achievetheiroptimum properties most aluminium-base alloys having high mechanical characteristics must be subjected to a series ofthermal and mechanical treatments and in particular a quenching operation, which generallyfollowsformation of a solid solution, at an elevated temperature usually at least equal to 430 C.

At the presenttime,the quenching operation is carried out using cold, warm or boiling water, depending on the particular circumstances involved, and the results obtained are not always the best possible compromise betweenthemechanical characteristics required, e.g. the dimensional stability of the components and their resistance to stress corrosion.

It is knownthatquenchingferrous alloys (carbon steels or alloy steels), which is often carried out in special oils, can also be performed in aqueous agents containing water-soluble organic polymers as additives.

Thus, in 1960, in a marketing brochure, Wyandotte Chemical Co. recommended using polyoxyalkylene glycols as an additive for quenching agents. The substance, which was identified bythe registered trade mark Pluracol V 10, had a molecular weight of from 25000 to 35000.

In 'Metals Handbook', it is stated thatthe addition of 0.01 % of polyvinyl alcohol to quenching water substantially increases the rate of cooling during the calefaction phase.

French Patent FR-A-1 384244 (equivalentto US-A-3 220 893) to Union Carbide Co. describes aqueous agents based on polyalkylene glycols, with the addition of anti-corrosion agents such as nitrite or borates.

French Patent FR-A-1 525 to B.A.S.F., A.G.

discloses the addition of a water-soluble polymer containing (-CO-NH-) groups, in a proportion of from 0.1 to 1% byweight German Patent Application DE-OS No.2349225 providesforthe addition of0.4to 10% byweight of a polyacrylic acid salt to water.

French Patent FR-A-2 316336 (equivalent to US-A-4 087 290)two to Houghton & Co. statesthatthe additive can also be a water-soluble salt of polyacrylic acid.

Finally US Patent No.3902929 assigned to Park Chemical Company proposes the use of polyvinyl pyrrolidone of a mean molecularweight of from 5000 to 400000, with the addition of nitrite andlor borax (Na2B407) as an anti-corrosion agent, togetherwith a biocide such as paraformaldehyde.

However, none ofthe above-identified documents provides or suggests the possibility of using such additivesforthe aqueous quenching ofaluminium- base alloys.

It is known that the operation of quenching aluminium alloys involves structural phenomena that are differentfrom those involved in quenching steels, and it is not possible to simply transfer the procedure from one situation to the other.

Efforts have therefore been made to develop a specific aqueous quenching agentforquenching aluminium alloys that gives the best possible compromise between the mechanical tensile strength characteristics and dimensional stability with respect to quenching ofthin or precision-die-cast components, and between the mechanical tensile strength characteristics and the residual stresses resulting from quenching of thick components.In dealing with thin or precision-die-cast components an essential criterion is isothermy of the surfaces and the possibility of reproducing the cooling effects; when dealing with thick components, an essential criterion is the attainment of accelerated reproducible cooling rates that permit of rapid quenching of the alloys in the critical quenching range (400 to 250"C) without causing excessive softening of the skin or surface layer of the metal, in particularattemperatures between 500 and 400"C. Such compromises are achieved by introducing suitably selected additives into the quenching water.

The present invention provides an aqueous quenching agentforquenching components of aluminium alloys, comprising, per litre of water, from 5to 35 g.

and preferably from 6 to 20 g. of polyvinyl pyrrolidone togetherwith an additive that, in the hot condition, causes instantaneous reversible precipitation ofthe polyvinyl pyrrolidone on the quenched component.

Use was made of the polyvinyl pyrrolidone produced by Badische Anilin and Soda Fabrik(BASF) underthe reference"K90", its mean molecularweightbeing about 700000 and in any case higherthan 400000, while it may attain 1000000.

The additive may be a water-soluble organic substance or a mineral saltthatissoluble in water and in aqueous solutions of polyvinyl pyrrolidone. However, sodium chloride has been found to be best suited to carrying the invention into effect. The additive may be introduced in a proportion offrom 5to 250 gA and, when the additive is NaCI, 100 to 250 gIl and preferably 150 to 200 g/l.

It was found that quenching a cylindrical testpiece of aluminium alloy by immersing it in the quenching agent according to the present invention had the following effects: (1) With concentrations of polyvinyl pyrrolidone (referred to in abbreviated form hereinafter as "PVP") ranging from 6to 20 g/l of water and with the addition of sodium cloride,the cooling effect achieved was (a) of uniformly retarded type with mean cooling rates that decrease in proportion to increases in the concentration of NaCI (up to 100 gill) and (b) of uniformly accelerated type (up to 2500C) with rising mean cooling rates, when the proportion of NaCI rises from 150 g/l to 250 g/l. In that case, the cooling rate is slightly higher (between 500 and 400"C) or lower (in the critical quenching range of 400 to 2500C) than that achieved by immersion in water at 600C, in a non-agitated condition.

Therefore, from 1 50 to 200 g/l of NaCI should be added tothe PVP solutions in orderto achieve cooling of the accelerated type, which is advantageous in respect of the compromise between mechanical properties and quenching deformation.

(2) When the component to be quenched is immersed, it is found thatthere is progressive and reversible precipitation of a thick insulating layer of PVP at the surface of the testpiece (beginning at the bottom), followed by rapid, complete re-dissolution of that layer atthe end of the cooling phase (at about 100 to 150 C) if the proportions of additives dissolved remain lowerthan or equal to 20 g/l of PVP and 250 g/l of NaCI.

(3) Agitation permits of an increase in the cooling rates of solutions containing 100 g/l of NaCI but it does not cause a significant acceleration in the cooling rate in solutions containing 150 to 200g/l of NaCI, which are therefore substantially insensitive to differences in agitation. On the other hand, it promotes the simultaneous precipitation of the PVP over the entire surface of the testpiece, which is a priori also favourableto reproducibilityofthetreatmentsandto minimizing residual stresses or deformation on components of large dimensions.

(4) With proportions of NaCI of 150 to 200 g/l, the rate of cooling is independent of the PVP content (between 6 and 20 g/l) and the state of surface oxidation of the metal. In addition, in contrast to cold water or a fortiori not water (60 C), the solutions containing PVP in a proportion of 12.5 gll + NaCI in proportions offrom 150to 200 gIl, which are prepared underthesame conditions from the same batch of basic substances, give reproducible cooling effects that are substantially insensitive to the temperature of the solution at from 15 to 25 C.

The following Tables set out the results of quenching tests carried out on a splittube of AU4G (2017) and on plate made of 7075, which show the attraction afforded by solutions of PVP + salt.

The testconditions are setout below: Optimum concentrations: aqueous solution of PVP in a proportion offrom Sto 20 gIl, preferably 1 0to 15 g/l, with the addition of 100 to 250 g/l and preferably from 120 to 200 g/l of NaCI.

EXAMPLE No.1 This test, which seeks to evaluate deformation due to the quenching operation, was carried out on a tube made of A1J4G (2017), with a length of 230 mm, # of 60 mm and a thickness of 2.5 mm, split along a generatrix with 25 mm between the lips so defined.

Thetubewas subjected to a solid-solution treatment lasting for 15 minutes at a temperature of 480 C.

Batches of4tubes were treated. They had been quenched by vertical immersion in a 100-litre tank without agitation. The amount by which the lips had come together (-) or moved apart (+) was then measured, at 5 points at spacings of 40 mm, and the average of the 5 measurements over each of the 4 tubes was used.

Comparative tests were also carried out, using waver at 200C and at 50 C, and air:

Deformation of tubes No Coooling Fluid rate in Averag Mini rate 1 2 4 3 Pvp 12.5 g/l+ 250 +0.11 o.oa 0.13 0.09 0.10 0.14 150 g NaCi PVP 12.5 g/l + 350 +0.18 0.10 0.17 0.08 0.14 +0.14/-0.22 20D q NsCl Air 1.1 +0.14 +0.08 +0.10 + 0.06 +0.095 0.02/0.18 Water 20 C 600 +0.58 +0.50 +0.30 + 0.61 +0.50 0.22/0.92 Water 600C 203 +0.26 +0.05 -0.03 + 0.02 +0.11 -0.09/0.30 EXAMPLE No. 2 Measurements were then made in respect of the influence ofthe quenching operation on the mechanical characteristics and deformation of metal plates made of 7075 (AZ8GU) measuring 400 x 400 x 8 mm, which were quenched by rapid vertical immersion in a 200-litre tank. The plates had been previously subjected to a solid-solution treatment for a period of 4 hou rs at a tem peratu re of 480"C.

Comparative tests were carried out by quenching with water at 200C and at 60 C. The results are as follows:

~ Deformation Quenching Elastic limit Breaking Elong fluid Rp 0.2, MPa stress ation "Sag" "Tile- Cooling Re, MPa A % I in rate 0CIs PVP at 12.5 g/l + 487 563 15.5 0.0 0.0 80 150 g NaC1 PVP at 12.5 g/l + 487 563 16.1 0.0 0.0 100 200 g NaCI Water 200C 498 568 11.6 1.5 1.5 200 Water 600C 486 559 12.5 5 7 60/110 "Sag" is longitudinal bowing; "Tile-type" is transverse bowing.

It appears therefore that this quenching agent considerably improves the compromise in regard to mechanical strength, deformation on quenching and residual stresses. It gives resu Its markedly better and more reproducible than those obtained using hot waterfor quenching.

Itwill be appreciated that the components of aluminium alloys quenched in the above-indicated agent containing NaCI must be subjected to a final rinsing with water in orderto prevent any risk of subsequent corrosion, however slight.

Although other salts such as borax (Na2B407) (upto 50 g/l) and sodium sulphate (to 80 g/l) give interesting results, only sodium chloride was such as to ensure both stability ofthe quenching agent (it does not cause irreversible precipitation of PVP) and an optimum compromise between the various characteristics involved.

Claims (8)

1. An aqueous quenching agent based on polyvinyl pyrrolidoneforquenching components of aluminium-base alloy that have been previously raised to a temperature that is at least equal to 430"C and comprising, per litre of water, from 5 to 35 of polyvinyl pyrrolidone and from 50to 250 g of an additive causing reversible precipitation of polyvinyl pyrrolidone at the surface ofthe components at the momentwhen theyare introduced into the quenching agent.

2. An aqueous quenching agent as claimed in Claim 1 containing from 6to 20 g/l of polyvinyl pyrrolidone.

3. An aqueous quenching agent as claimed in Claim 1 or 2, in which the polyvinyl pyrrolidone has a mean molecular weight at least equal to 400000.

4. An aqueous quenching agent as claimed in Claim 3, in which the polyvinyl pyrrolidone has a mean molecularweightthatattains 1000000.

5. An aqueous quenching agent as claimed in any preceding claim, in which the additive is sodium chloride in a concentration offrom 100 and 250 g/l.

6. An aqueous quenching agent as claimed in Claim 5, in which the sodium chloride concentration is 1 to 250 g/l.

7. An aqueous quenching agent as claimed in Claim 1 substantially as hereinbefore described.

8. Aluminium-base-alloy components that have been raised to a temperature of 430"C and subsequently quenched using an aqueous quenching agent as claimed in any one of the preceding claims.