• PDF / 1,715,618 Bytes
• 9 Pages / 593.972 x 792 pts Page_size
• 33 Downloads / 249 Views

DOWNLOAD

REPORT

---

IN the fields or applications of high-precision machining, semi-conductor manufacturing processes, two conjugated rotating components, and so on, that involve the generation of heat during the operation, the demand for achieving high dimensional accuracy under the influence of thermal heat increases continuously. Alloys with low thermal expansion coefficients are suited for such applications. The Invar effect was first discovered by Guillaume and Chevenard in 1987. The Invar effect of an alloy relates to its magnetic property that counteracts the normal thermal expansion to achieve near zero thermal expansion coefficient, a so-called magnetostriction property.[1,2] However, the Invar effect of an alloy only appears within a certain range of temperature, around 293 K to 473 K (20 °C to 200 °C), in which the upper temperature limit is affected by the Curie temperature LI-HAO CHEN and ZONG-PEI LIU, Graduate Students, and YUNG-NING PAN, Professor, are with the Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan. Contact e-mail: [email protected] Manuscript submitted September 25, 2015. Article published online May 16, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A

(Tc), a transition temperature from ferromagnetic to paramagnetic. Due to its unique magnetostriction property, Ni is a primary element in achieving a low thermal expansion coefficient in the Fe-base alloys.[3] The minimum coefficient of thermal expansion occurs at Ni content within the range of 34 to 36 pct for temperature below 373 K (100 °C).[4,5] On the other hand, the element cobalt (Co) has a similar effect as Ni on the thermal expansion coefficient, due to the facts that Co has a much higher Curie temperature [1403 K (1130 °C)] than Ni [631 K (358 °C)] and both Co and Ni have similar melting points; 1768 K vs 1728 K (1495 °C vs 1455 °C). When some of the Ni is replaced by a suitable amount of Co, the thermal expansion coefficient can be further lowered at a constant NiE of 35 to 36 pct (NiE[4] = pctNi + (0.82 to 0.84) pctCo). The equivalent ratio for the substitution of Co for Ni is about 0.8 to 1.0.[6–8] Inverse Ni segregation and C concentration away from equilibrium (normally it is higher than the equilibrium value) are generally encountered in the alloys in the as-cast state. As a result, a values are adversely affected.[5,9–11] The primary purposes of this research are to investigate the effect of homogenization heat treatment on the carbon content dissolved in the matrix, the VOLUME 47A, AUGUST 2016—4137

degree of Ni segregation, and also the a value, with the purpose to establishing the optimal heat treatment conditions for attaining the lowest possible a values, and also to evaluate the dimensional stability of low thermal expansion cast irons with various heat treatment conditions.

II.

EXPERIMENTAL PROCEDURES

The alloy design is presented in Table I, and the homogenization heat treatment conditions are given in Table II.

Table I.

The Chemical Composition of the Alloys (Wt Pct)

Heat No.

C

Si

Ni

Co

Fe

I II

1.8 2.0

1.8 1.8