Hot deformation behavior of a new type of M3 : 2 high speed steel with niobium addition made by spray forming was investigated based on compression tests in the temperature range of 950–1150 °C and strain rate of 0.001–10 s−1. A comprehensive constitutive equation was obtained, which could be used to predict the flow stress at different strains. Processing map was developed on the basis of the flow stress data using the principles of dynamic material model. The results showed that the flow curves were in fair agreement with the dynamic recrystallization model. The flow stresses, which were calculated by the comprehensive constitutive equation, agreed well with the test data at low strain rates (⩽1 s−1). The material constant (α), stress exponent (n) and the hot deformation activation energy (QHW) of the new steel were 0.00615 MPa−1 4.81 and 546 kJ • mol−1 respectively. Analysis of the processing map with an observation of microstructures revealed that hot working processes of the steel could be carried out safely in the domain (T = 1050–1150 °C, e = 0.01–0.1 s−1 ) with about 33% peak efficiency of power dissipation (η). Cracks was expected in two domains at either lower temperatures (<1000 °C) or low strain rates (0.001 s−1 ) with different cracking mechanisms. Flow localization occurred when the strain rates exceeded 1 s−1 at all testing temperatures.

Hot Deformation Behavior and Processing Map of Spray Formed M3: 2 High Speed Steel

To develop advanced high speed steel with high vanadium content, a kind of vanadium content 9% high speed steel in the form of circular ring was spray formed under certain thermal conditions. The carbides topography of as-cast and as-sprayed 9V steel were observed. The effects of post thermo-mechanical processing (hot isostatic pressing and hot forging) and heat treatment on the microstructure and carbides distribution were investigated. The spray formed 9V steel typically consists of fine, homogeneous and angular-shaped MC carbides with diameters ranging from 1 to 2 μm. Spray formed 9V steel shows an excellent hot workability following increase of forging ratio due to fine homogeneous carbides. After heat treatment, spray formed 9V steel is composed of MC carbides, martensite and retained austenite. The hardness value of heat treated materials is inversely proportional to the volume fraction of residual austenite. Fine dispersed MC carbides have significant influences on the dislocation motion of spray formed 9V steel.

Microstructure and carbides characteristics of spray formed 9V high vanadium steels

In order to study the hot workability and to optimize the processing parameters for spray formed FGH4095 superalloy, thermal compression tests for spray formed FGH4095 superalloy have been finished by using a Gleeble 1500 thermal simulated test machine at the strain rates of 0. 01—10. 0 s −1 and temperatures of 1050—1140 °C. The effects of strain rate and deformation temperature on the true stress-true strain curves and microstructure evolution were investigated. The results show that the generation of dynamic recrystallization (DRX) depends sensitively on deformation temperature. When the temperature was lower than 1080 °C, long and narrow necklace grains were shown in the microstructure. When the temperature increased to 1140 °C, new recrystallization grains were generated. The size and shape of γ′ precipitates in the grains have a very important effect as factors of hindering sufficient migration of dislocations on plastic deformation. The result of thermal processing map is in accord with the microstructure observation, and the best material thermal processing temperature is above 1128 °C.

Chuan-shui Sun

Papers

Microstructural Characterization of Spray Formed FGH4095 Superalloy During Hot Deformation