CN103091469B - Method for calculating allowable stress of plastic metal casting materials - Google Patents

Abstract

The invention belongs to the technical field of centrifugal casting, and particularly relates to a method for calculating the allowable stress of a plastic metal casting material. According to the method, the allowable stress value is the ratio value of yield strength to a safety factor N; the safety factor N is the sum of the product and a basic safety factor n of the casting material, the product is of the increase coefficient k of the safety factor of the casting material at the temperature of more than 300 DEG C and the maximum between the difference (between the casting temperature T and 300 DEG C) and zero DEG C and a basic safety factor n of the casting material; when the plastic metal casting material is carbon cast steel or ductile cast iron, the temperature T is set within the range of 20-800 DEG C; and when the plastic metal casting material is chromium molybdenum steel, the temperature T is set within the range of 20-900 DEG C. The method for calculating the allowable stress of the common plastic metal materials for centrifugal casting moulds under the condition of high temperature provides a reliable scientific basis for the design of the centrifugal casting moulds, and simultaneously builds a foundation for quantitative analysis of safety accidents of centrifugal casting mould factures.

Description

A kind of computing method of plastic metal mold material permissible stress

Technical field

The invention belongs to centrifugal casting technique field, be specifically related to a kind of computing method of plastic metal mold material permissible stress.

Background technology

In centrifugal casting, plastic metal is conventional mold material, comprises carbon cast steel, chromium molybdenum steel, spheroidal-graphite cast iron.For these plastic metal materials, its permissible stress [σ] is yield strength σ _0.2divided by safety factor N, i.e. [σ]=σ _0.2/ N.But because the mechanical behavior under high temperature test of mold material is less, basic data is comparatively deficient, causes hot conditions lower mold material yield intensity σ _0.2data are incomplete, and the value of safety factor N never finds reliable method to try to achieve simultaneously.Therefore in the design process of casting mold, selection to plastic metal mold material, often just using the ambient temperature mechanical properties of plastic metal mold material under static condition as casting mold design with reference to foundation, and casting mold is worked under special high temperature, high velocity environment, compare with ambient temperature mechanical properties, the mechanical behavior under high temperature of material can reduce a lot, even if by considering that some remainings select centrifugal casting mould material, sufficient not often.Not yet there is at present the permissible stress data of mold material under hot conditions.

From casting handbook (mechanical engineering society's foundry meeting. cast handbook the 2nd volume, cast steel [M]. Beijing: the .2011 of the China Machine Press third edition; Mechanical engineering society's foundry meeting. casting handbook the 1st volume, cast iron [M]. Beijing: the .2011 of the China Machine Press third edition), the mechanical behavior under high temperature that can find various plastic metal materials is:

(1) mechanical behavior under high temperature of carbon cast steel: the yield strength of carbon cast steel, at 20 to 400 ℃ of slow decreasings, has higher yield strength and length growth rate (σ in the time of 400 ℃ _0.2=160MPa, δ=15%), between 400～500 ℃, decline obviously, in the time of 500 ℃, also have certain yield strength and length growth rate (σ _0.2=130MPa, δ=26%), in the time of 550 ℃, still have certain yield strength and length growth rate (σ _0.2=120MPa, δ=23%).ZG230-450 in carbon cast steel and ZG270-500 are the conventional trades mark of centrifugal casting mould material.

(2) mechanical behavior under high temperature of chromium molybdenum steel: chromium molybdenum steel yield strength in the time of 20 ℃ to 400 ℃ changes little, yield strength slow decreasing within the scope of 400 ℃～550 ℃, has very high yield strength and length growth rate (σ in the time of 550 ℃ _0.2=260MPa, δ=24%), between 550～650 ℃, decline and accelerate, in the time of 650 ℃, still have quite high yield strength and length growth rate (σ _0.2=205MPa, δ=30%).ZG20CrMo in chromium molybdenum steel, ZG25CrMo are the conventional trades mark of centrifugal casting mould material.

(3) mechanical behavior under high temperature of spheroidal-graphite cast iron: the yield strength of a. ferrite ductile cast iron, at 20 ℃ to 400 ℃ slow decreasings, has higher yield strength and length growth rate (σ in the time of 400 ℃ _0.2=255MPa, δ=12%), between 400～500 ℃, decline obviously, in the time of 500 ℃, still have certain yield strength and length growth rate (σ _0.2=145MPa, δ=8%); In the time of 600 ℃, still have certain yield strength and length growth rate (σ _0.2=90MPa, δ=9%), when higher than 600 ℃, yield strength declines significantly, and in the time of 700 ℃, yield strength is lower, and length growth rate improves (σ _0.2=45MPa, δ=16%); B. the yield strength of pearlite ductile iron, at 20 ℃ to 450 ℃ slow decreasings, has higher yield strength and length growth rate (σ in the time of 450 ℃ _0.2=375MPa, δ=12%), between 450～550 ℃, decline larger, in the time of 550 ℃, still have higher yield strength and length growth rate (σ _0.2=237MPa, δ=17.7%), in the time of 600 ℃, still have certain yield strength and length growth rate (σ _0.2=165MPa, δ=37.2%), when higher than 650 ℃, yield strength declines significantly, the lower (σ of yield strength in the time of 700 ℃ _0.2=73MPa, δ=17.6%).The main matrix of QT450-10(in spheroidal-graphite cast iron is ferrite) with the main matrix of QT500-7(be ferrite and pearlite) be the conventional trade mark of centrifugal casting mould material.

From above-mentioned data, can find out, no matter plastic metal material is carbon cast steel, chromium molybdenum steel, or spheroidal-graphite cast iron, at 20 ℃, within the scope of 400 ℃, the variation of their mechanical behavior under high temperature is all little, and when more than 400 ℃, its yield strength is all accelerated to decline.

For plastic metal material, permissible stress [σ] is yield strength σ _0.2divided by safety factor N, i.e. [σ]=σ _0.2/ N, but due to hot conditions lower mold material yield intensity σ _0.2data deficiency, safety factor N cannot accurately try to achieve, make the permissible stress of plastic metal material under hot conditions be difficult to accurate acquisition, cause centrifugal casting designer to carry out by rule of thumb the design of centrifugal casting mould, cannot meet the requirement in actual production process.

Summary of the invention

Technical matters to be solved by this invention is to provide a kind of reliable plastic metal mold material permissible stress computing method, selects mold material that the foundation of permissible stress comparatively is reliably provided during for centrifugal casting.

For solving the problems of the technologies described above, technical scheme of the present invention is:

Computing method for plastic metal mold material permissible stress, described plastic metal mold material permissible stress [σ]=σ _0.2/ N; Wherein, σ _0.2for yield strength, N is safety factor, and the computing formula of safety factor N is:

N=n+k×max[(T-300),0]

In formula: n is the basic safety factor that mold material is got;

K is 300 ℃ when above, mold material safety factor increase progressively coefficient, get 0.01;

T is mold temperature.

Preferably, when described plastic metal mold material is carbon cast steel or chromium molybdenum steel, described basic safety factor n is 3; When described plastic metal mold material is spheroidal-graphite cast iron, described basic safety factor n is 4.

Preferably, when described plastic metal mold material is carbon cast steel or spheroidal-graphite cast iron, the scope of mold temperature T is set as 20～800 ℃; When described plastic metal mold material is chromium molybdenum steel, the scope of mold temperature T is set as 20～900 ℃.

The invention provides the computing method of the conventional plastic metal material permissible stress under hot conditions of a kind of centrifugal casting mould, the design that the method is centrifugal casting mould provides scientific basis comparatively reliably, broken away from the full Design Mode by rule of thumb of centrifugal casting mould material, simultaneously for the security incident quantitative test of hydro-extractor veining is laid a good foundation.

Embodiment

Embodiment 1

The present embodiment is chosen carbon cast steel ZG230-450 as mold material, and mold temperature T setting range is 20～800 ℃, and the process of permissible stress of calculating this mold material is as follows:

(1) completion of carbon cast steel ZG230-450 yield strength data

From Wan Jiali chief editor's < < electromechanical engineering metal material handbook > >, can find ZG230-450 the yield strength test figure of 20 ℃～600 ℃, in Table 1.

Table 1 carbon cast steel ZG230-450 mechanical behavior under high temperature

In the present embodiment, the maximal value of mold temperature T is taken as 800 ℃, has only provided 200 ℃ of above temperature intervals and be the yield strength data corresponding to each temperature of 50 ℃ on handbook, lacks 600 ℃ of above yield strength data, therefore yield strength σ _0.2data incomplete, can the yield strength data of 400 ℃～600 ℃, do equation of linear regression to ZG230-450, obtain 400 ℃～800 ℃ scopes, the pass of arbitrary temp and yield strength is

σ _0.2=-0.5T+433

Because of R ²=0.9796, coefficient R=0.9898, therefore illustrate that the accuracy of above-mentioned equation of linear regression is very high.The temperature interval providing due to handbook is 50 ℃, still by this temperature interval, calculates: the yield strength in the time of 650 ℃ is

σ _0.2=-0.5×650+433=108MPa；

In like manner: when T is 700 ℃, σ _0.2for 83MPa; When T is 750 ℃, σ _0.2for 58MPa; When T is 800 ℃, σ _0.2for 33MPa.

Within the scope of 20 ℃～350 ℃, can pass through method of interpolation, try to achieve the relation of temperature and yield strength.In the time of 20 ℃～200 ℃, available following formula calculates

$\left[{\mathrm{\&sigma;}}_{0.2T}\right]={\mathrm{\&sigma;}}_{0.2\left(200\right)}-\frac{{\mathrm{\&sigma;}}_{0.2\left(200\right)}-{\mathrm{\&sigma;}}_{{0.2T}_1}}{200-T_1}\&CenterDot;(200-T)$

σ _0.2Tyield strength during for temperature T, T ₁for room temperature, σ _0.2T1for temperature T ₁time yield strength.In the time of 100 ℃, yield strength is

$\left[{\mathrm{\&sigma;}}_{0.2T}\right]={\mathrm{\&sigma;}}_{0.2\left(200\right)}-\frac{{\mathrm{\&sigma;}}_{0.2\left(200\right)}-{\mathrm{\&sigma;}}_{0.2T_1}}{200-T_1}\&CenterDot;(200-T)=230-\frac{230-255}{200-20}\&CenterDot;(200-100)$

$=244\left(\mathrm{MPa}\right)$

In the time of 250 ℃, σ _0.2be 200 ℃ with the arithmetic mean of 300 ℃, i.e. 230MPa.

So far, completion yield strength σ _0.2data.

(2) calculate the safety factor N of carbon cast steel ZG230-450

N=n+k×max[(T-300),0]=3+0.01×max[(T-300),0]

Wherein, n is that 3, k is 0.01, works as

When T is 20 ℃～300 ℃,

N=3+0.01×max[(T-300),0]=3+0.01×0=3.0；

When T is 350 ℃,

N=3+0.01×max[(350-300),0]=3+0.01×max[50,0]=3+0.01×50=3.5；

In like manner can obtain: when T is 400 ℃, N=4.0; When T is 450 ℃, N=4.5; When T is 500 ℃, N=5.0; T is 550 ℃, N=5.5; T is 600 ℃, N=6.0; T is 650 ℃, N=6.5; T is 700 ℃, N=7.0; T is 750 ℃, N=7.5; T is 800 ℃, N=8.0.

(3) calculate the permissible stress of carbon cast steel ZG230-450

According to permissible stress formula [σ]=σ _0.2/ N, can calculate carbon cast steel permissible stress [σ]: T under different mold temperature T is 20 ℃, [σ] is 85MPa; When T is 100 ℃, [σ] is 81MPa; When T is 200 ℃, [σ] is 77MPa; When T is 250 ℃, [σ] is 77MPa; When T is 300 ℃, [σ] 77MPa; When T is 350 ℃, [σ] is 66MPa; When T is 400 ℃, [σ] is 58MPa; When T is 450 ℃, [σ] is 47MPa; When T is 500 ℃, [σ] is 38MPa; When T is 550 ℃, [σ] is 27MPa; When T is 600 ℃, [σ] is 23MPa; When T is 650 ℃, [σ] is 17MPa; When T is 700 ℃, [σ] 12MPa; When T is 750 ℃, [σ] is 8MPa; When T is 800 ℃, [σ] is 4MPa.

Embodiment 2

The present embodiment is chosen spheroidal-graphite cast iron QT500-7 as mold material, and mold temperature T setting range is 20～800 ℃, and the process of permissible stress of calculating this mold material is as follows:

(1) completion of spheroidal-graphite cast iron QT500-7 yield strength data

According to the full experimental study > > (< < heat processing technique > > the 2nd phase in 2000) adopted, that bend the paper < < spheroidal-graphite cast iron high temperature elastic plastic mechanical properties of golden rosy clouds of business, can find QT500-7 mechanical behavior under high temperature (in Table 2).

The mechanical behavior under high temperature of table 2 spheroidal-graphite cast iron

In the present embodiment, the maximal value of mold temperature T is taken as 800 ℃, therefore yield strength σ in table 2 _0.2data cover the scope of temperature T of whole casting mold, but between adjacent temperature data, interval is excessive, needs by the yield strength σ of method of interpolation completion QT500-7 _0.2.200 ℃ of above temperature intervals are set as to 50 ℃, calculate yield strength σ corresponding to different mold temperature T _0.2:

When T is 100 ℃,

$\left[{\mathrm{\&sigma;}}_{0.2T}\right]={\mathrm{\&sigma;}}_{0.2\left(200\right)}-\frac{{\mathrm{\&sigma;}}_{0.2\left(200\right)}-{\mathrm{\&sigma;}}_{0.2T_1}}{200-T_1}\&CenterDot;(200-T)=317-\frac{317-365}{200-18}\&CenterDot;(200-100)$

$=343\left(\mathrm{MPa}\right)$

In the time of 300 ℃, σ _0.2the arithmetic mean 311MPa of yield strength when yield strength and 400 ℃ while being 200 ℃; In the time of 250 ℃, σ _0.2the arithmetic mean 314MPa of yield strength when yield strength and 300 ℃ while being 200 ℃; In like manner:

When T is 350 ℃, σ _0.2for 308MPa; When T is 450 ℃, σ _0.2for 290MPa; When T is 550 ℃, σ _0.2for 221MPa; When T is 650 ℃, σ _0.2for 119MPa; When T is 750 ℃, σ _0.2for 57MPa.

(2) calculate the safety factor N of spheroidal-graphite cast iron QT500-7

N=n+k×max[(T-300),0]=4+0.01×max[(T-300),0]

Wherein, n is that 4, k is 0.01, works as

When T is 20 ℃～300 ℃,

N=4+0.01×max[(T-300),0]=4+0.01×0=4.0

When T is 350 ℃,

N=4+0.01×max[(350-300),0]=4+0.01×max[50,0]=4+0.01×50=4.5

In like manner can obtain: when T is 400 ℃, N=5.0; When T is 450 ℃, N=5.5; When T is 500 ℃, N=6.0; When T is 550 ℃, N=6.5; When T is 600 ℃, N=7.0; When T is 650 ℃, N=7.5; When T is 700 ℃, N=8.0; When T is 750 ℃, N=8.5; When T is 800 ℃, N=9.0.

(3) calculate the permissible stress of spheroidal-graphite cast iron QT500-7

According to permissible stress formula [σ]=σ _0.2/ N, the permissible stress of spheroidal-graphite cast iron QT500-7 [σ] in the time of can calculating different mold temperature T: when T is 18 ℃, [σ] is 91MPa; When T is 100 ℃, [σ] is 85MPa; When T is 200 ℃, [σ] is 79MPa; When T is 250 ℃, [σ] is 79MPa; When T is 300 ℃, [σ] is 78MPa; When T is 350 ℃, [σ] is 68MPa; When T is 400 ℃, [σ] is 61MPa; When T is 450 ℃, [σ] is 53MPa; When T is 500 ℃, [σ] is 46MPa; When T is 550 ℃, [σ] is 34MPa; When T is 600 ℃, [σ] is 24MPa; When T is 650 ℃, [σ] is 16MPa; When T is 700 ℃, [σ] is 9MPa; When T is 750 ℃, [σ] is 7MPa; When T is 800 ℃, [σ] is 4MPa.

The computing method of the conventional plastic metal material permissible stress under hot conditions of centrifugal casting mould that the embodiment of the present invention provides, for the design of centrifugal casting mould provides scientific basis comparatively reliably, broken away from the full Design Mode by rule of thumb of centrifugal casting mould material, simultaneously for the security incident quantitative test of hydro-extractor veining is laid a good foundation.

It should be noted last that, above embodiment is only unrestricted in order to technical scheme of the present invention to be described, although the present invention is had been described in detail with reference to preferred embodiment, those of ordinary skill in the art is to be understood that, can modify or be equal to replacement technical scheme of the present invention, and not departing from the spirit and scope of technical solution of the present invention, it all should be encompassed in the middle of claim scope of the present invention.

Claims (2)

1. computing method for plastic metal mold material permissible stress, described plastic metal mold material permissible stress [σ]=σ _0.2/ N; Wherein, σ _0.2for yield strength, N is safety factor, it is characterized in that, the computing formula of safety factor N is:

N=n+k * max[(T-300), 0], wherein:

N is the basic safety factor that mold material is got, and when described plastic metal mold material is carbon cast steel or chromium molybdenum steel, described basic safety factor n is 3; When described plastic metal mold material is spheroidal-graphite cast iron, described basic safety factor n is 4;

K is 300 ℃ when above, mold material safety factor increase progressively coefficient, get 0.01;

T is mold temperature.

2. the computing method of plastic metal mold material permissible stress according to claim 1, is characterized in that, when described plastic metal mold material is carbon cast steel or spheroidal-graphite cast iron, the scope of described mold temperature T is set as 20～800 ℃; When described plastic metal mold material is chromium molybdenum steel, the scope of described mold temperature T is set as 20～900 ℃.