In 1931, the Indian National Congress adopted it to don India's pre-Independence flag as an emblem of the anti-colonial movement. But in July 1947, just before independence, the charkha was replaced with the Ashokan wheel (chakra) in the design of India's national flag.

When did the Chakra replaced the charkha spinning wheel in the national flag?

On 22nd July 1947 "Chakra" between the National Flag had replaced spinning wheel which represents the law.

When Chakra in the national flag replaced the spinning wheel?

On July 22, 1947, the Indian national flag was officially hoisted. After the partition of India in August 1947, the colours of newly independent India's flag remained the same, but its original spinning wheel was replaced by a blue chakra—the Dharma Chakra.