CN103091469A - 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 the 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 the mold material of commonly using, and 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 the plastic metal mold material, often just with the ambient temperature mechanical properties of plastic metal mold material under static condition as the 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, and is even if by considering that some remainings select the centrifugal casting mould material, sufficient not often.The permissible stress data of mold material under hot conditions not yet arranged at present.

From the 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 has higher yield strength and length growth rate (σ at 20 to 400 ℃ of slow decreasings in the time of 400 ℃ _0.2=160MPa, δ=15%), descend obviously between 400～500 ℃, also have certain yield strength and length growth rate (σ in the time of 500 ℃ _0.2=130MPa, δ=26%), still have certain yield strength and length growth rate (σ in the time of 550 ℃ _0.2=120MPa, δ=23%).ZG230-450 in carbon cast steel and ZG270-500 are the trades mark commonly used 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, and yield strength slow decreasing in 400 ℃～550 ℃ scopes has very high yield strength and length growth rate (σ in the time of 550 ℃ _0.2=260MPa, δ=24%), descend between 550～650 ℃ and accelerate, still have quite high yield strength and length growth rate (σ in the time of 650 ℃ _0.2=205MPa, δ=30%).ZG20CrMo in chromium molybdenum steel, ZG25CrMo are the trades mark commonly used 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 has higher yield strength and length growth rate (σ at 20 ℃ to 400 ℃ slow decreasings in the time of 400 ℃ _0.2=255MPa, δ=12%), descend obviously between 400～500 ℃, still have certain yield strength and length growth rate (σ in the time of 500 ℃ _0.2=145MPa, δ=8%); Still have certain yield strength and length growth rate (σ in the time of 600 ℃ _0.2=90MPa, δ=9%), yield strength descends significantly higher than 600 ℃ the time, 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%), descend between 450～550 ℃ larger, still have higher yield strength and length growth rate (σ in the time of 550 ℃ _0.2=237MPa, δ=17.7%), still have certain yield strength and length growth rate (σ in the time of 600 ℃ _0.2=165MPa, δ=37.2%), yield strength descends significantly higher than 650 ℃ the time, 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 trade mark commonly used of centrifugal casting mould material.

Can find out from above-mentioned data, no matter plastic metal material is carbon cast steel, chromium molybdenum steel, or spheroidal-graphite cast iron, in 20 ℃ to 400 ℃ scopes, the variation of their mechanical behavior under high temperature is all little, and in the time of more than 400 ℃, its yield strength is all accelerated to descend.

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 can't accurately try to achieve, make the permissible stress of plastic metal material under hot conditions be difficult to accurate acquisition, cause the centrifugal casting designer to carry out by rule of thumb the design of centrifugal casting mould, can't satisfy 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 reliably is provided during for centrifugal casting.

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

A kind of computing method of plastic metal mold material permissible stress, described plastic metal mold material permissible stress [σ]=σ _0.2/ N; Wherein, σ _0.2Be 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 more than 300 ℃ the time, the mold material safety factor increase progressively coefficient, get 0.01;

T is mold temperature.

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

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

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

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 " electromechanical engineering metal material handbook can be found ZG230-450 the yield strength test figure of 20 ℃～600 ℃, sees 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 temperature interval more than 200 ℃ on handbook and be the yield strength data corresponding to each temperature of 50 ℃, lacks the yield strength data more than 600 ℃, therefore yield strength σ _0.2Data incomplete, can do equation of linear regression the yield strength data of 400 ℃～600 ℃ 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 is therefore illustrate that the accuracy of above-mentioned equation of linear regression is very high.The temperature interval that provides due to handbook is 50 ℃, and still calculate by this temperature interval: 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 _0.2Be 83MPa; When T is 750 ℃, σ _0.2Be 58MPa; When T is 800 ℃, σ _0.2Be 33MPa.

In 20 ℃～350 ℃ scopes, 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

$\left[{\mathrm{\σ}}_{0.2T}\right]={\mathrm{\σ}}_{0.2\left(200\right)}-\frac{{\mathrm{\σ}}_{0.2\left(200\right)}-{\mathrm{\σ}}_{{0.2T}_1}}{200-T_1}\·(200-T)$

σ _0.2TYield strength during for temperature T, T ₁Be room temperature, σ _0.2T1Be temperature T ₁The time yield strength.In the time of 100 ℃, yield strength is

$\left[{\mathrm{\σ}}_{0.2T}\right]={\mathrm{\σ}}_{0.2\left(200\right)}-\frac{{\mathrm{\σ}}_{0.2\left(200\right)}-{\mathrm{\σ}}_{0.2T_1}}{200-T_1}\·(200-T)=230-\frac{230-255}{200-20}\·(200-100)$

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

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 that under different mold temperature T, carbon cast steel permissible stress [σ]: T is 20 ℃, and [σ] is 85MPa; When T was 100 ℃, [σ] was 81MPa; When T was 200 ℃, [σ] was 77MPa; When T was 250 ℃, [σ] was 77MPa; When T is 300 ℃, [σ] 77MPa; When T was 350 ℃, [σ] was 66MPa; When T was 400 ℃, [σ] was 58MPa; When T was 450 ℃, [σ] was 47MPa; When T was 500 ℃, [σ] was 38MPa; When T was 550 ℃, [σ] was 27MPa; When T was 600 ℃, [σ] was 23MPa; When T was 650 ℃, [σ] was 17MPa; When T is 700 ℃, [σ] 12MPa; When T was 750 ℃, [σ] was 8MPa; When T was 800 ℃, [σ] was 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

The paper " experimental study of spheroidal-graphite cast iron high temperature elastic plastic mechanical properties " (" heat processing technique " the 2nd phase in 2000) of justice complete in the business, the golden rosy clouds of bow can be found QT500-7 mechanical behavior under high temperature (seeing 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, the interval is excessive, needs the yield strength σ by method of interpolation completion QT500-7 _0.2Temperature interval more than 200 ℃ is set as 50 ℃, calculates yield strength σ corresponding to different mold temperature T _0.2:

When T is 100 ℃,

$\left[{\mathrm{\σ}}_{0.2T}\right]={\mathrm{\σ}}_{0.2\left(200\right)}-\frac{{\mathrm{\σ}}_{0.2\left(200\right)}-{\mathrm{\σ}}_{0.2T_1}}{200-T_1}\·(200-T)=317-\frac{317-365}{200-18}\·(200-100)$

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

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

When T is 350 ℃, σ _0.2Be 308MPa; When T is 450 ℃, σ _0.2Be 290MPa; When T is 550 ℃, σ _0.2Be 221MPa; When T is 650 ℃, σ _0.2Be 119MPa; When T is 750 ℃, σ _0.2Be 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 was 18 ℃, [σ] was 91MPa; When T was 100 ℃, [σ] was 85MPa; When T was 200 ℃, [σ] was 79MPa; When T was 250 ℃, [σ] was 79MPa; When T was 300 ℃, [σ] was 78MPa; When T was 350 ℃, [σ] was 68MPa; When T was 400 ℃, [σ] was 61MPa; When T was 450 ℃, [σ] was 53MPa; When T was 500 ℃, [σ] was 46MPa; When T was 550 ℃, [σ] was 34MPa; When T was 600 ℃, [σ] was 24MPa; When T was 650 ℃, [σ] was 16MPa; When T was 700 ℃, [σ] was 9MPa; When T was 750 ℃, [σ] was 7MPa; When T was 800 ℃, [σ] was 4MPa.

The computing method of the centrifugal casting mould that the embodiment of the present invention provides plastic metal material permissible stress under hot conditions commonly used, 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, laid a good foundation for the security incident quantitative test of hydro-extractor veining simultaneously.

It should be noted last that, above embodiment is only unrestricted in order to technical scheme of the present invention to be described, although with reference to preferred embodiment, the present invention is had been described in detail, 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 breaking away 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 (3)

1. the computing method of a plastic metal mold material permissible stress, described plastic metal mold material permissible stress [σ]=σ _0.2/ N; Wherein, σ _0.2Be 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;

K is more than 300 ℃ the time, the 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 was carbon cast steel or chromium molybdenum steel, described basic safety factor n was 3; When described plastic metal mold material was spheroidal-graphite cast iron, described basic safety factor n was 4.

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