FUSIBLE ALLOY FOR PRESSURE RELIEF DEVICES

Abstract

The present invention relates to a fusible alloy for pressure relief devices (PRDs), and the present invention provides a fusible alloy for PRD, the alloy comprising 29.0 to 33.0% by weight of Bi, 14.0 to 21.0% by weight of Sn, 2.0 to 5.0% by weight of In, and substantially Pb for the balance. The fusible alloy for PRD of the invention has a melting point appropriate for use according to the 110° C.-grade standards, and has excellent wettability upon melting.

Description

TECHNICAL FIELD

The present invention relates to a fusible alloy for pressure relief devices (PRDs), and more particularly, to a fusible alloy for PRD, which is appropriate for use according to the 110° C.-grade standards and has excellent wettability upon melting.

BACKGROUND ART

Pressure relief devices are safety devices attached to gas cylinder valves in automobiles and the like, which devices play the role of preventing gas explosion by rapidly discharging gases inside the cylinder to the outside when the temperature of the ambient environment rises abnormally to above a specific temperature, such as at the time of fire.

Accordingly, a conceptually ideal PRD is expected to completely cut off the outflow of gases up to a specific temperature, and when the specific temperature is reached, to discharge the entire amount of the gases within a short time. Thus, only those verified products which have undergone rigorous tests are put into use.

In this light, for example, the European standards stipulate a use of alloys having a melting point acceptable for the 110° C.-grade (110±10° C.), to provide against safety accidents involving gas explosions.

DISCLOSURE

As such fusible alloys for PRD, ternary alloys such as Bi—Pb—Sn alloys, Bi—Cd—Sn alloys and the like have been traditionally used. However, the wettability upon melting of these alloys is not sufficient, and the alloys have had limitations in rapidly discharging gases inside the cylinder to the outside.

Thus, it is an object of the present invention to solve such problems of the prior art, and to provide a new fusible alloy for PRD, comprising a quaternary alloy which ensures excellent wettability upon melting at a specific temperature, and at the same time, meets the requirements stipulated by the European standards as the 110° C.-grade.

DESCRIPTION OF DRAWINGS

FIG. 1 shows photographs showing the wetting angles of the fusible alloy for PRD according to the present invention and of a conventional fusible alloy for PRD, as measured in a wettability test.

FIG. 2 shows scanning electron microscopic (SEM) photographs of exemplary reinforcement materials used in the preparation of the fusible alloy according to the present invention.

BEST MODE

In order to achieve the above-described object, according to the present invention, there is provided a quaternary fusible alloy for PRD, comprising 29.0 to 33.0% by weight of bismuth (Bi), 14.0 to 21.0% by weight of tin (Sn), 2.0 to 5.0% by weight of indium (In), and substantially lead (Pb) for the balance.

The present invention chooses to use a quaternary alloy because the low-melting point alloying elements, namely, Pb, Bi, Sn and In, themselves have low melting points, and also because when these elements form a polynary alloy, the alloy may have a eutectic point, thus it being possible to render the melting point of the alloy even lower. For example, in the case of a Pb—Sn alloy, which is a representative solder material, while the melting point of Pb is 327° C., and the melting point of Sn is 232° C., an alloy of Pb with 61.9% of Sn has a melting point of 183° C. at the eutectic point. Based on this tendency, the melting point of an alloy can be further lowered by increasing the number of alloying elements to systems of ternary, quaternary and the like.

Meanwhile, according to the present invention, the results of thermodynamic calculations were used as the basis of the primary alloy designs for appropriate composition ratios of the alloying elements. In other words, the melting points of ternary, quaternary and quinary alloys of various compositions were predicted based on the calculations using thermodynamic data. Then, a quaternary system of Bi—Sn—In—Pb that was considered to be an optimum was selected to constitute the alloy of the present invention, and the composition was determined to fall within a specific range through actual experimentation.

Thus, in the present invention, the composition ratios of the alloy-constituting elements were determined to be 29.0 to 33.0% by weight for Bi, 14.0 to 21.0% by weight for Sn, 2.0 to 5.0% by weight of In, and substantially Pb for the balance. If the respective component elements are contained in the alloy of the invention in amounts beyond the above-described composition ranges, the melting point (in particular, the melting start point) may be lowered to below 100° C., thus causing the alloy to melt at a temperature below the critical temperature; or the melting point may be raised to above 120° C., thus inhibiting efficient discharge of gases at an appropriate temperature and subsequent prevention of safety accident such as gas explosion. Then, such alloys are deprived of the function as the 110° C.-grade fusible alloys for PRD, and are undesirable.

Hereinafter, the present invention will be described in more detail.

Evaluation of Main Properties of Fusible Alloy for PRD

The properties that are necessary for an alloy to be used as a fusible alloy for PRD, include melting point, wettability, PRD durability, and the like.

Evaluation of Melting Point In order to prepare alloys of desired compositions, the composition ratios by weight were calculated, and the elements were respectively weighed. Then, the elements were mixed and melted together to form fusible alloys, and then, mother alloy ingots of the respective compositions, each weighing about 1 kg, were prepared. Subsequently, each fusible alloy was placed on a substrate made of brass and maintained at a desired temperature for 10 minutes using glycerin solution as a heating medium, and then, the presence or absence of melting of the fusible alloy was verified. The compositions of the alloys were analyzed by energy dispersive spectroscopy (EDS) (an average of three points), and the evaluation results are presented in Table as the melting point according to the composition of an alloy.

	TABLE 1
		Melting
	Composition (ratio	start	Melting
	by weight, wt %)	point	end point
	Pb	Bi	Sn	In	(° C.)	(° C.)
Example	1	48.85	29.38	18.85	2.92	113.0	118.0
	2	48.36	29.25	19.46	2.94	109.0	117.0
	3	46.81	30.96	18.28	3.95	106.0	116.0
	4	45.77	30.87	19.87	3.50	112.0	120.0
Comp. Example	1	42.15	28.41	25.97	3.47	98.4	117.0
	2	17.31	71.08	9.90	1.70	98.0	100.0
	3	22.62	60.71	13.08	3.59	99.2	107.0
	4	30.43	47.19	16.03	6.36	99.4	116.0

As can be seen from Table 1, the alloys having the compositions according to the present invention (Examples 1 to 4) were all found to have melting points that exceed 100° C., thus satisfying the European standards relating to the 110° C.-grade fusible alloys for PRD. On the other hand, the alloys of Comparative Examples all had melting start points of 99.4° C. or lower, and were found to be undesirable since melting of the alloys would start at a temperature lower than the minimum required temperature (in the case of the 110° C.-grade, the allowed temperature range is 110±10° C., and thus, 100° C. is the minimum required temperature).

Evaluation of Wettability

In order for a fusible alloy to form strong bonding to the internal walls of a PRD and the interior reinforcement materials, the fusible alloy should have excellent wettability. A wettability test is performed by melting alloys of the same weight and measuring the area occupied by the alloy, or by inspecting the cross-section of a specimen that has been melted and quenched, and measuring the wetting angle. A smaller wetting angle means better wettability. FIG. 1(a) in the upper photograph shows a conventional Bi—Cd—Sn ternary alloy, and FIG. 1(b) in the lower photograph shows a Pb—Bi—Sn—In quaternary alloy (Example 4) according to the present invention.

Referring to FIG. 1, the wetting angle of the Bi—Cd—Sn ternary alloy was measured to be 52.78°, while the wetting angle of the Pb—Bi—Sn—In quaternary alloy of the present invention was measured to 47.25°. Thus, the wetting angle of the alloy of the present invention was confirmed to be smaller.

Evaluation of PRD Durability

To evaluate the durability of a PRD, a PRD needs to be actually produced from a fusible alloy. A PRD employing a composite material is formed by filling the interior of the PRD body with a fusible alloy mixed with the reinforcement materials as shown in FIG. 2. The reinforcement materials used in the present invention were stainless steel spheres having a diameter of about 600 mm (FIG. 2(a)) and stainless steel cylinders having a length of about 900 mm (FIG. 2(b)), which were used in a mixture. The experimental results indicate that when the reinforcement materials were mixed at a ratio of about 50:50, the durability was found to be optimum.

As such, PRDs were produced using the quaternary fusible alloys according to the present invention, and a durability test was performed. According to the test conditions stipulated by the ISO International Standards, the test temperature was 91° C., the test pressure was 325 bars, and the test duration was 500 hours. After 500 hours of the test, if the fusible alloy inside the PRD did not flow out, the fusible alloy was considered to be acceptable. The test procedure is as follows.

(1) A sample PRD is placed in a test block, which is in turn placed in an oven.

(2) A pressure of 325 bars is applied onto the sample PRD by means of air.

(3) The temperature in the oven is adjusted to 91° C., and then the sample PRD is maintained therein for 500 hours.

(4) No deformation or operation of the sample PRD is allowed for the duration of 500 hours.

When the test was performed in the above-described manner, those PRDs filled with the fusible alloys of the present invention were all found to be acceptable, and were confirmed to have durability that is at least equivalent to that of existing alloys.

INDUSTRIAL APPLICABILITY

As discussed in the above, the quaternary fusible alloy for PRD of the present invention has a melting point that is appropriate for use according to the 110° C.-grade standards, and has excellent wettability as demonstrated by the small wetting angle upon melting. Thus, PRDs formed of the fusible alloy of the invention can be expected to discharge gases within a short time, and the fusible alloy of the invention is a material which can satisfactorily replace conventional ternary alloys.

Claims

1. A fusible alloy for pressure relief devices, the alloy comprising 29.0 to 33.0% by weight of bismuth (Bi), 14.0 to 21.0% by weight of tin (Sn), 2.0 to 5.0% by weight of indium (In), and substantially lead (Pb) for the balance.