The Bombay Stock Exchange benchmark Sensex surged by 170 points in opening trade on Tuesday on fresh buying by funds. Sensex was trading at 16,462, up by 106 points when reports last came in.Similarly, the wide-based National Stock Exchange index Nifty gained 28 points at 4,928 points.Brokers said that firming trend in Asian markets triggered buying in IT, metal and teck stocks.Asian markets were trading in the positive zone following overnight gains in the US markets.Hong Kong's benchmark Hang Seng rose 1.18 per cent, while Tokyo's Nikkei rose by 1.88 per cent in morning trade. Shanghai was up by 1.35 per